The bug
Some items are not adjusted properly if the player sneaks. This seems to affect all "flat" items, but affects carpets and slabs as well. Full blocks are not affected.
MC-162402
Some items are not adjusted properly in third-person mode if the player holding them is sneaking
Resolved
Fixed
4
Anonymous
Confirmed
Important
Rendering
19w39a, 19w40a
19w42a
Retrieved
Can confirm. This seems to happen with all items but full blocks.
The issue for tridents is MC-162374, I'll keep that separate as tridents are kinda special with how they're rendered. Also, the issue with charging while sneaking is a bit older, see MC-125204.