If you have a trapdoor above you and under you, you are too high to fit betwwen without sneaking.
BUT if you open the trapdoor under you and the close it again, your legs will be in the trapdoors, and you won't start sneaking.
Also, you won't be able to start sneaking.
I think related to: MC-148749 -Related to> MC-148390
Automatic sneaking would be okay in that situation, but maybe causing a worse case, wich is unsolvable now -> Won't be fixed (Suffocating issues)
But if fixable, I would be happy to see it fixed