The8BitMonkey

Confirm for 22w46a

Confirmed for 20w29a

Figured it would be best to include a video of the bug 

It still does not work even when removing the b at the end

Was successful 

/execute as @e run data merge entity @s {IsBaby:1}

Was unsuccessful

/execute as @e run data merge entity @s {IsBaby:0}

As a test that it wasn't my system at fault, I opened the world on a second PC and this was the result after summoning 10 or 12 pillagers, the spike started once again after the iron golem registered the pillager but couldn't attack

Hi, sorry for the delay in reply, had to return to work and not been able to check messages.

I've attached the world file generated in 1.16 Pre-Release 5 (latest as of posting) and still experiencing the bug.

Did a few tests myself and it would seem that the issue does lie with the Golem as the spikes only start to happen once the iron golem takes notices of the Pillagers, while the pillagers are just trying to attack the golem but before it notices them there are no spikes but as soon as the iron golem starts to track one of the pillagers but can't attack them the spikes start to go up.

Here is a screenshot of the issue just before I exited the game and saved it to attach here:

If it's at all helpful my system specs are:

• AMD Fx 8350 @ 3.51GHz

• Sapphire Nitro+ RX 590 8GB

• 24GB DDR3 Ram

Using the latest MC launcher and latest snapshot (pre-release) version

Once the TPS gets into the red as shown in the video mob movements becomes jittery and almost non existent, yes my FPS is fine and I'm able to move around the area but I'm no longer able to interact with my surroundings. 

Confirmed 17w17b

Confirmed 17w17a
When going to Chat Settings there is now a grayed button with "Narrator Not Available" written in it.

Confirm for 17w16a

Confirmed for 17w16a

(Previous snapshot message would appear but now it doesn't show anything on the screen)

Tested and can confirm this to be the case.
When in spectator made if a parrot was flying to you when switching mode it will land on your player also if you fly over a parrot while in spectator mode it will also go to the players shoulder, in both cases it will turn invisible.

Video deminastrating the issue.
https://gfycat.com/RespectfulBitesizedAlbacoretuna

Confirmed 17w14a

Confirmed 17w14a

Confirmed 17w13b

When wearing the Elytra and climbing a ladder, if you press space bar you will fall at a high speed down the ladder.

Confirmed 17w13b

Confirmed for 17w13b

Confirmed 17w13a

When on the players shoulder the parrot seems to no longer be an entity, when showing the hitbox (F3+B) on mobs and items one does no appear around the parrot on the shoulder.

Edit:
It would seem that the target selector for the parrots on the shoulder are within the players NBT not through the parrot entity.

/testfor @a {ShoulderEntityLeft:{id:"minecraft:parrot"},ShoulderEntityRight:{id:"minecraft:parrot"}}

gif of bug in action

I was able to re-produce this bug by simply throwing stacks of items to the ground, in my tests it happens rarely for small amounts of stacks but the more you throw down the more likely it is to happen.