In my point of view, choosing to incorporate an SD card into Internal Storage is handing over the integrity of the whole Internal Storage to the SD card. If the card becomes corrupt, which I think it will most probably sooner than the hardwired storage chips of the phone, with the only question being when exactly, it may take the whole Internal Storage down with it.
It seems the OS has strict checks in place for the state of the SD card. That’s a good thing.
I guess the possibility to use an SD card this way dates back to when smartphones didn’t have enough storage for anything … but the Fairphone 2? At 32 GB internal?
What’s your use case (honest question)? Perhaps it could be addressed differently …