invisible blocks in survival

I have a witherskull farm with crusher. So I went to my farm and some iron blocks were missing (iron trench). But there is still a block with a hitbox of a sign, ladder or door (see pic).
EDIT: there were iron blocks instead of the invisble blocks. And you can just walk true them.
EDIT 2: When I re-logged into minecraft, the hitbox changed to a normal block hitbox