If you try and place trapdoors of any kind on the top section of tall grass then they will break the grass but stay hovering there, updating the block beside it also doesn't cause it to break. Breaking a block that a trapdoor was placed on also doesn't break the trapdoor.
It also happens in 1.14.4 so I'm not sure if this is a bug or intended.
Thank you for your report!
We're actually already tracking this issue in MC-83069, so I resolved and linked this ticket as a duplicate.
However, that ticket has already been resolved. That means that this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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