Digest and analysis of original description by GoldenHelmet:
Block data in the affected world shifted +64 Y resulting in the following problems:

• Double chests changed to single chests
• Chest contents wiped
• Item frames wiped
• Banners changed to solid gray/black
• Beds all turned red
• Dragon head block changed to a skeleton skull
• Wither skeleton skull blocks changed to skeleton skulls
• Potions in cauldrons changed to water
• Armor stands and minecarts have disappeared
• Pets and animals suffocated
• Blocks pushed by sticky pistons no longer retract (compare MCPE-111632)
• Returning from the nether through a nether portal puts player inside solid blocks underground.

All of these effects are easily explained by the fact that entity and tile entity position data did not change with block data. The game therefore created or "read" default or <data: 0> state/contents for blocks that should have tile entity data (e.g. empty chests and item frames, red beds, normal skeleton skulls)--compare MCPE-78279. Entities ended up relatively 64 blocks lower, mostly inside solid blocks. The armor stands and minecarts should be able to be found underground.

If you read this read everything, you might just help... That is for the other people not mojang. These are for mojang and the other people. When i updated my game i went into my favorite world to get new blocks, but i saw the double chests were not double chests anymore, they single chests put next to each other, i then went to check my other chests, when i opened it it was wiped all my chests are wiped, basically all i have left is the items in my inventory the decorated banners are now grayish black, item frames that had items on them are wiped, my world's Y coordinate was in the 60s it's now in the 130s, all my pets suffocated, infact i think all the animals in the world suffocated, the Y coordinate change must have suffocated my pets and all the animals on my world, the dragon head on my world is now an skeleton skull, blocks in front of sticky pistons no longer stick when the sticky piston is activated, unless you break the block(s) in front of the sticky piston(s) and replace it, the minecarts, gone. Signs are wiped (no letters on them), I went into the nether portal to check if it's alright, and yes, it's perfectly fine, but, when i went back, i found my self stuck in bedrock, on the Y coordinate 50 my beds, are from the color they used to be to red, oh, and the wither skulls i placed are also skeleton skulls now so, if you ask me, i think all the heads in minecraft that is placed in players worlds on the 1.16.230.54 version of minecraft (which is the one this huge bug is created on) are now, skeleton skulls. Potions in cauldrons are now water placed armor stands, with armor, or items, are gone idk if there's more, but, if someone reads this, go and test it, and then comment on this and add stuff that is also corrupted, and mojang, fix this as quickly as possible i will have to delete my 488mb world, unless you can recover all of my stuff which i doubt you can do, if it's not recovered, i will delete my world, and start all over, and next time, test the update, before launching it, pls.. Like for this type of problem.. Pls, Fix it as soon as possible. I just forgot to take an screen shot of the wither skull - skeleton skull, I'm not gonna do that, I'm not supposed to be on my phone right now, i have limited time now, and if someone had wither skeleton skulls on their worlds, and updated to the 1.16.230.54 version of minecraft, go and check if it's skeleton skulls, and comments on this, same with the other heads. Sorry for the typos if there is... There is alot of screen shots, and 1 screen recording.