In 1.14, crawling (and forced sneaking) were introduced, where the player's hitbox (and therefore animation) would be moved and scaled to fit the available space depending on the size of the new space. However, this only appears to work for when the top half of the player experiences a change - equivalent setups for the legs do not have an effect at all, leading to strange behaviour like being unable to sneak manually (or enter a buggy mechanical-only, non-visual form of sneaking).

Expected behaviour would be that, assuming space is available, the player entity would be moved upwards and then enter a sneaking or crawling pose depending on the new space available.