The bug
Trapped chests seem to have a gray square on the front which is usually obscured by the lock, but makes itself seen when the chest is opened. This square does not exist on normal chests.
How to reproduce
Open a normal chest
Exit the chest
Before the chest closes, pay close attention to the region usually covered by the lock
Rinse and repeat, but with the trapped chest
Expected results
This gray square would not exist on the trapped chest texture.
Actual results
It does.
I believe this is intentional, it resembles a button that you activate when you open the chest.