When you sneak on bottom half slabs or the bottom step of a stair block you can't crouch into a 1.5 block tall gap anymore even if the hitbox should technically fit in there.

Standing in a 2 blocks high space, and jump up while sneaking can have the player pushed out of the space.

However, why won't this happen when jumping while crawling under a full block? Maybe this is key to the problem.

A way to fix this is to fakely set the height of the player to 0.8 while the height of the bounding box is still 1.5. However this can only be a temp fix and should be able to be switched off in configuration as it may break other modded things.

Btw shouldn't the title be "Player cannot sneak under 1.5 block spaces if the block above the player is a full block"? The currently used title is truly misleading.

I will be checking into this to see what I can do

It seems to be something to do with how minecraft handles players inside of opaque full blocks. The game is trying to force the player out of the block as if they are being forced into the space. I'm still trying to figure out a way to get it working however but this may take awile longer

I managed to get it working without any changes. Can you by chance show me how you are doing it?

the issue seems to depend on if there is a solid block above your head or not, I made this little video in singleplayer to give you a better idea of what's going on:
https://www.youtube.com/watch?v=y1v3PyuXrtU

Ah I see exactly what you mean now. I will be trying to fix this now