Signs stuck on walls under water can block pathfinding for bees.
Repro steps
1. Create a 2x2 hole, 3 deep.
2. Put signs on the walls of the middle layer and fill the hole with water.
3. Spawn a bee in the waterfilled block under a sign.
4. Observe that often the bee will get stuck until it suffocates, it's pathfinding stuck at the sign.
Note
I was not able to reproduce this beehaviour without filling the hole with water.
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Can confirm in 20w51a.
Can confirm in 21w03a.
Can confirm in 21w05a.

I can confirm this "beehavior," and it seems that bees count signs as full blocks, similar to mobs walking straight into holes if there are open trapdoors above it. Any other floating mob (allays, phantoms, etc.) will most likely do the same.