There appears to be a bug where, on login to a realm, BDS or singleplayer worlds, the player will fall through the block that was previously below them.

Steps to Reproduce:

1. Stand on a trap door when logging out
2. Log back in
3. Fall

Observed Results:
When logging out on top of a trapdoor you will be guaranteed to drop down when logging back in.

Expected Results:
You should log back in on top of the trapdoor.

Blocks Tested:

Trapdoors - Fall through

Slabs (upper or lower) - Fine

Carpet on top of torch - Fine

Notes:

This may be limited to trapdoors but I mention blocks in general as it also seems to have happened in SlackLizards recent video. There are also bug reports for falling through bedrock but those seem to be chunk loading issues so I didnt just add to those.