The Bug:
Signs normally require some kind of visual attachment to a supporting block in order to exist in the world, however, they can be placed on top of doors causing them to appear floating.
Steps to Reproduce:
Place down a door and obtain a sign.
Attempt to place the sign on the top face of the door.
Take note as to whether or not signs can be placed on top of doors.
Observed Behavior:
Signs can be placed.
Expected Behavior:
Signs would not be able to be placed on the top of doors.
Attachments
Comments
Can confirm in 21w03a.
Can confirm in 21w05b.
Can confirm in 21w06a.
Can confirm in 1.16.5 and 21w08b.
Can confirm in 21w11a.
Can confirm in 1.17.
Can confirm in 1.17.1.
Can confirm in 21w41a. The expected behavior here would be that signs cannot be placed on top of doors.
Can confirm in 1.18.2 and 22w18a.
I think it's intentional the way it is

I agree with @Phonia.
Possibly is, but you'll need a Mojang source saying so, otherwise you're just flooding the comment section with empty statements.
Can confirm in 20w51a.