There is no trinkets backpack slot when in creative mode. The border is there but it is hollow and there is nothing intractable inside. This also results in a crashing behaviour when switching between game modes:
When wearing a backpack in the trinkets slot in survival, then switching to creative and opening the actual creative mode inventory (pressing e) in single player results in the 'kicked from the server' crash

The versions of the mods I am using are:
Inmis 2.8.1
Trinkets 3.10.0