Activating a trapdoor one block above the ground while standing "under"/in it does NOT activate crouching movement for the player.
This functionality does exist on Java edition and is apparently considered intentional.
The only way to enter 1 block high passageways on bedrock edition is by swimming or elytra flying into an opening of that size.
Linked issues
is duplicated by 12
BDS-18330
Player no longer sneaks when going under a trapdoor
Resolved
MCPE-75696
trapdoor dont push you down when closing
Resolved
MCPE-78967
unable to use trapdoors to crawl
Resolved
MCPE-81389
trapdoor intersects character rather than crawl
Resolved
MCPE-82472
Bug with Trapdoors
Resolved
Comments 9
The bigger problem at hand is why does mojang/Microsoft insist on the divergent path between Bedrock and Java.
the crawl state is something g that needs to be addressed. Either allow i players to crawl and add in the proper support for Bedrock or fix the effective bug that you can be in a swimming state outside of water. This discrepancy makes a whole lot more issues in game play and other bugs like MCPE-35808. Where there is an improper save/reload when in a single block high space.. ie swimming outside of water.
Sneaking over 1 block high not working is not a bug because Bedrock Edition never had this function. It is suitable that it is a parity to if it is equivalent to Java Edition.
Bedrock does not support that functionality. That feature hasn't been implemented in bedrock edition...