You can no longer place signs on the side of scaffolding.

iPhone XR, minecraft multiplayer

When a sign is placed on the side of scaffolding, placing or destroying anything beside the block the sign is in will cause the sign to break. 

How to recreate. Place a scaffolding block. Put a sign on the side of the scaffolding. Place anything next to the block the sign is occupying. The sign will break. 

How to recreate. Place a scaffolding block. Place anything beside the area you intend on putting sign at. Place the sign on said block on the scaffolding. Destroy the object you placed earlier. The sign will break.