World occasionally fails to load correctly

Not really, no. It's sorta similar in that sometimes chunks don't render, but with MC-70850 it only happens when the chunk is partially obscured. This is not the case here, from what I can tell the reason for chunks not rendering is the chunk rendering system being overwhelmed.

Also MC-70850 is happening for a lot longer than just 1.15

So far, I couldn't find reproduction steps, creating another world with the same seed will not have the same missing chunks at the same location, but when I encountered another missing chunks area, I did some basic testing; the video is rather lengthy (~15 minutes), but maybe showing the issue extended may give some of you some pointers where to look, what could be the cause: https://www.youtube.com/watch?v=GiF7IdENN30

Some general observations (numbers are video timestamps which show what's mentioned):

• With F3 turned on, you'll see as Biome "Waiting for chunk..." when you're in the affected missing chunks region (whole video)

• Reloading all chunks (F3+a) does not do anything

• The missing chunks go all the way down to Bedrock level; from underneath, you can see the actual amount of missing chunks (2:45)

• The - at least from some angles - not rendered chunks make out a straight line (4:15), although the not rendered chunks are not actually affected and pop back into existence, dependent on how you move your camera; the actually affected missing chunks will make a hole all the way down, whereas the only seemingly missing chunks won't, when you look down (7:45)

• If you move the player camera, neighbouring chunks outside of the "straight chunks line" may not appear as well and pop back and forth in/out of existence (4:30, 9:00)

• Mobs you push, spawn, or walk into a missing chunk will freeze and shake (10:00, 11:10, 11:20, 11:40, )

• The same goes for items; you can see this best with items if you activate hitboxes with F3+b (12:45)

• You cannot place any blocks inside the missing chunk, also not falling blocks (0:45, 10:45);
  it is possible to setblock a block, however, it will not show up visually (1:05, 10:55)

• If you place water at the border of the affected chunk, it will not flow inside of the missing chunk (13:15)

• If you open the world to LAN and get a second account in, this second account will also see the very same missing chunks and experience everything the same

  [media]

• The player does not fall through the missing chunks, they will stand like on invisible blocks (12:20); walking inside a missing chunk is very slow, as if on soulsand/inside cobweb

• Despawning of items inside missing chunks is not affected

• Changing the render distance (16 > 32 > 10) (4:00, 9:05) does not affect the missing chunks rendering, still more than the actually affected missing chunks will show up as visually missing from some angles

• Relogging fixed the missing chunks for me; the water I placed before at the border of the missing chunk flowed into the previously affected chunk, the stone I setblocked before could now be visually seen

• While relogging fixes this issue, this is not the case if you have a 2nd account around the area, and then relog; as long as the host/main account is in the world, this issue will persist, at least in Open to LAN worlds, for also other players (unsure about actual servers atm)

I also found this bug! I recorded it immediately and the footage shows me trying to interact with the world + animals.

https://www.youtube.com/watch?v=a42rUVph5BI

Seems I've found the same bug too, snapshot on 21w10a on Windows 10 system. I needed milk to remove a Bad Omen status, and walked/ran in survival mode from my base a few hundred blocks away as seen in the location -338 79 -223 and found the chunk. Walking across the invisible blocks is extremely slow. I took a few screenshots and one image of the F3 text shows "Waiting for chunk..." message. Saving/exiting the game and reloading it resolved rendering the chunk properly.

Same error on 
OS : Windows 10 Pro 64-bit
Java version : 11.0.10 64-bit
CPU : AMD Ryzen 5 5600x CPU @ 4.6 GHz
GPU : NVIDIA GeForce RTX 3070
Drivers : GeForce 461.72
RAM: 16GB

I has this happen to me once while there was a skeleton in the chunk, it kept shooting at me even though I couldn't see it or shoot back

This is another example of it happening

This happened to me back on 11a but havent experienced it so far on 13a

Can confirm in 21w14a.

21w15a both with caves and cliffs datapack, and without the datapack.

I think this is the same bug that I experienced, uploaded video of it and screenshot of my video settings in 21w15a ~PIAF7B.mp4