Support/interface layer welded to flat surface

I’m printing this bridge-type structure. I have tried both Support/raft base and support/raft interface set to PLA Support filament, and also with just the interface.

I can never remove the last bit of support interface filament.

Maybe I’m not supposed to - or am I doing something wrong?

When I chose PLA Support for interface, it popped up a prompt offering to change various settings for me. As a newcomer, this made little sense to me (this isn’t really newcomer-friendly expecting me to know the answer to the prompt), but since it suggested it I thought it must be a good idea so I chose it. I guess I’ll try reverting these settings.