When using a custom bang to search the UK Parliament's House of Lords members, I get inconsistent behaviour with the placement of the exclamation mark depending on what the custom bang is. If it is "ld" (lower-case L), then I can place the exclamation mark on either side: !ld or ld! If it is "lord" (also lower-case L), then I can only place the exclamation mark at the beginning for the custom bang to work.
My understanding is that the exclamation mark should be placeable on either side of the bang, custom or otherwise.
!ld! Settings

!ld! Testing
!lord! Settings

!lord! Testing