The bug
While the crawling/swimming pose has a smooth animation which is played when transitioning between it and other poses, no such animation is played for the crouching/sneaking pose. This conflicts with what is seen in first-person view: the transition between the two camera positions is smooth (previously reported under MC-48401, which says Won't Fix but was eventually fixed some time in 1.13 or 1.14 anyway; also see MC-90591).
How to reproduce
There are easier ways to test this, but I used a dedicated setup in case this issue is valid for sneaking caused by blocks but not the controls:
Place a piston which will push a block down on a standing player, forcing them into crawl mode, when triggered, and will retract it when triggered a second time
Enter third person mode and trigger the mechanism, and observe the smooth animation that results
Replace the block beign pushed with a top slab
Note the instant pose change
Expected results
The standing-to-crouching and crouching-to-standing pose changes would be smooth, much like the standing-to-crawling and crawling-to-standing changes are.
Actual results
The change is instant, which is inconsistent with both the crawling transitions and what is seen in first-person view.
Further notes
Animated GIFs for comparison:
Standing<-> crawling:
[media][media]
Standing<-> crouching:
[media][media]
Attachments
Comments 3
This is pretty similar to texture reports, where those are either rejected or fixed (like the inventory door items, which got changed due to a report here). This would fall into the grey area of inconsistencies, so it's something that may get ot not get fixed.
This is a feature request, not a bug.