Mach

Read the report notes please. I documented seed and steps to recreate the issue already.

World on OneDrive linked:
https://1drv.ms/u/s!Avql4hN5V3T1g0bVHAi5-nUxYULp?e=uUirVa

https://mega.nz/file/YUIBVTpB#yjGOehrel-LUbPh3X5vrMXS2mKhAmBauyTrHik64ruI World download can be found here. Re-uploading the original video clip too.

Further testing shows that setting world spawn successfully solves the problem if a block is available at the world spawn y level for the mob to tp to. I tested by creating a normal and a flat world instead of the attached void world.

Prior to setting world spawn, the normal world shulkers disappeared. I could not find them or get them to say hi using /execute as @e[type = shulker] run say hi. Unclear where they went.

After setting worldspawn to the players present location on the surface, Shulkers consistently appeared at the user set world spawn.

On the flat world, shulkers went to the void and then teleported to the surface. After setting the world spawn, they teleported to the user defined overworld world spawn. If all blocks between world spawn and the void are removed using a fill command, the shulkers will appear in the void as if world spawn was not set. Thank you to Zetec for pointing this out.

Linked is a 30 min video showing the wander bug and the reload bug.  The first half of gameplay shows the wander behavior, where new mobs generate when old mobs wander away.  Then the video shows what happens if you move out of the chunk and close the game, then reopen the game and move back into the chunk.  The spawner generates a new set of mobs.  All told, 36 mobs were generated in 30 minutes from 1 spawner.

https://youtu.be/5_jM51gmXgY

In testing, one spawner generated 43 mobs. Activated one stray spawner in chamber #8 in survival.  Watched a mob spawn and then switched to creative.  Watched the next 6 mobs generate and walk off the side to wander away from the spawner.  Tp'd to y=100 to keep the chunk loaded.  Spawner went into cooldown between 45 and 60 minutes later.  During testing, the world was closed repeatedly to allow the mobs to be counted using rBedrock.  Reloading the world caused a new set of mobs to generate from the spawner. 

43 mobs were generated.  The spawner NBT data only recorded the last 4 mobs generated.  Spawner located at 140, -2 119 in the reported seed.  

I tested on 1.21.0.26 using Reed's instructions and spawns occurred as expected.   Test world attached.

Pressure plates should also block spider spawns due to the same mechanic but do not in the 1.21 preview.  Efforts to fix this bug should look at pressure plates too.

Seeing this after the 1.20.70 update.  
Villager trading hall with 10 established librarians that are working and linking as expected.  New villager (zillager from a spawner) cured, locked, and brought in from over 100 blocks away.   Librarians in a 1x1 cell and standing on lecterns (again other 10 are fine).  After trading, 1st 10 reset as expected - new librarian does not reset even after waiting multiple in game days.  Villagers are within 20 blocks of 20-30 barrels.  There are no fisherman in the trading hall.  Breaking barrels closest to bugged librarian causes bugged librarian to reset trades.  Confirmed twice by trading and breaking barrels causing bugged librarian to reset.  Also noted that breaking the bugged librarian's workstation does not cause normally expected storm clouds.  Bugged librarian will also not link to a newly placed lectern when no other lectern is present.  All other librarians are trading as expected so no cross linking.  No other village within 100 blocks.  Bed and lectern used for curing that were located 100 blocks away were also broken and removed as a precaution.  No change in bugged librarian after breaking remote bed and lectern. 

The seed is 6340053227976873656

So I was experiencing this bug more frequently recently in both directions.  I dialed my render distance back to 12 chunks.  I haven't gotten the overworld to nether bug since making the change.  I still occasionally get the nether to overworld bug.

Falling in the nether after experiencing the portal bug.

After upgrade to 1.20.31, I'm now experiencing this bug moving from the overworld to the nether.  Minecraft locks up on building terrain.  I've left it for up to 10 minutes waiting for it to load.  When I restart the game, I am falling in the nether void at my x,z overworld coordinates.  I fall continuously past -32,000 block if I leave myself fall that long.  If I have elytra on, I can fly back towards x=0 or z=0 and when I hit chunks that are previously loaded, I receive the message that I fell out of the world and die losing all my gear.  This is the 4th time this has happened.  Really makes me not want to play any longer.

Bug happened again while another user was streaming.  Clips attached.  Server had 5-10 users actively playing.  User complains about server lag shortly before bug occurs.  User had not experienced this bug before.  This was nether->overworld travel.

https://clips.twitch.tv/SweetSpeedyChinchillaImGlitch-oAZ-45p1WV2hre3N

https://clips.twitch.tv/FrigidSpinelessCrabDancingBaby-jZY2wliLZ2sAhQNG

Here's a link to the world.  It's 1GB in size.  https://drive.google.com/file/d/1Z_sJJ0STFKsusYNx-bCKvKEEkWQECDhM/view?usp=sharing

Please post here when you have retrieved it.

Here's a couple of clips of it occurring during a live stream by another player, Skyfire. 

https://clips.twitch.tv/TrappedWanderingCockroachMoreCowbell-Ze5h1nTQTab-Q9Ww

https://clips.twitch.tv/TrappedEntertainingPorcupineGingerPower-nJdeETw0F1NlVCLd

I can provide a link to this world download but is there a way that I can send it to you without posting it here?  BTW, it's 1GB.

@growlingbrain It may help if you go to the coordinates in your destination dimension and clear an area.  I was teleporting into solid rock and dying before I understood what was happening.  I was teleporting from the bottom of the Nether to the Overworld when I would get stuck on Building Terrain.  So, the failed teleport would put me very low in the Overworld in solid rock.  I cleared out an area at my nether portal coordinates in the overworld and built a bubble elevator to the surface.  I also left a chest with supplies to help with trip back to my base.  It doesn't fix the problem but I don't lose my armor anymore.

Upgraded to 1.19.51, started new world with seed 1056.  In creative mode, teleported to Stronghold at 38 -15 -1051.  Enabled and entered portal to enter End.  Command killed the dragon and ender pearled through the generated outer island portal.  Teleported to -3756 69 1366 .  Laid down 2 blocks entering the portal, laid down two rail, placed a minecart, then summoned a shulker.  Pushed the shulker by breaking the minecart through and ender pearled after it.  Arrived at obsidian platform but shulker \ were missing.  World export is attached with player on obsidian platform after completing the above steps.   -3756 69 1366 is the affected gateway - sorry didn't catch that you wanted the player at that location.

Note:  This streamer (https://youtu.be/B5eHSRSnisY?t=1866) shows a player who was able to successfully move a shulker by having another player wait on the main island to keep the chunks loaded while pushing the shulker through at the end island portal while in survival.  I attempted the same method by setting a ticking area.  The 1st attempt failed but then I was able to get a shulker through but removing the ticking area afterwards did not reverse the solution.  In other words, once a ticking area was set and removed then I was able to move shulkers consistently.  Possible an initialization problem?

No I never saw a crash in my testing.  Running on Windows 10 though -  not Android as reported in MCPE-165496.