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:

1. 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
2. Enter third person mode and trigger the mechanism, and observe the smooth animation that results
3. Replace the block beign pushed with a top slab
4. 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:

Standing<-> crouching: