Just realized that this is a duplicate of https://bugs.mojang.com/browse/MCPE-102371. Sorry!

When you attempt to place a sign in a block without a non full hitbox, the block disappears..

Steps to Reproduce:

1. Place down a block without a full hitbox.
2. Place down a sign on a block directly adjacent to the non full hitbox block. If you try with fence, you can place the sign on the block below the fence. If you try with a slab, you can put a block directly next to the slab. Try to place a sign on that block. Those are a couple of examples. Basically, just try to place a sign in the same block as the block without a full hitbox.

Observed Results:

The stair disappears when the sign is placed.

Expected Results:

Sign cannot be placed.