Hello! I have a similar issue as someone else in #22 in 1.12.2 [sit-1.12.2-v1.3.jar]. I initially sent a comment on that issue explaining this bug, but eventually decided to make it its own, more detailed issue instead, since the bugs we encountered may not be the same.

This bug seems to occur on a mod called Mo' Bends. Could this be an overall incompatibility for custom animation mods?

Since Mo' Bends is allegedly a client-side mod, this bug might cause some odd exploits, especially in Multiplayer, if someone were to have Mo' Bends installed while the rest don't. (I have no friends to test this thesis.)

The bug seems to occur when there is no floor below over a certain vicinity. Leaves are immune to this logic, though...?

Example 1: Introduction
https://user-images.githubusercontent.com/87086653/180611204-86f413e6-0594-40dc-a0b8-d5cf8450b6f1.mp4

Example 2: The Bug's Limits
https://user-images.githubusercontent.com/87086653/180611212-3a4d7ddf-933c-409c-ac97-5aed254a343e.mp4

Example 3: Exception - Leaves
https://user-images.githubusercontent.com/87086653/180611273-9390edf6-ec9d-4174-a4b3-f7e7f1d48ef9.mp4

I am aware this bug might need further documentation, but that's specific every case I can think of showcasing. I hope the footage helps!

This has nothing to do with Mo' Bends, I can reproduce all of the shown situations just fine with just Sit installed. Though I'm afraid to say, that due to the age of Minecraft 1.12.2, I am no longer supporting my mods for that version, thus this won't be fixed.

Understandable. Have a nice day!