SD cards formatted as internal storage (phone storage) have recently caused various issues with FP3s. I always recommend to format SD cards as portable/external storage instead. However, I only ever formatted my own as external storage right away, so I have no experience with what to pay attention to when switching them from external to internal. I’m mentioning this because just yesterday, the switch seems to have caused problems to one Fairphoner.
EDIT: The aforementioned problem with the switch from internal to external storage has been solved (see same link). After the switch, the former internal SD card needed to be removed in Settings > Storage in order to make the system fully realize and implement the switch.