Welcome to the community 
While I cannot determine a specific reason, we keep encountering issues here with SD cards fomatted as “phone storage” (or “internal storage” or “extended internal storage”). This format allows to save apps on the SD card, but keeps creating all sorts of issues like photos not being saved.
If there isn’t any more specific approach for a solution by anyone else, I would strongly recommend to re-format the card to “portable storage” (“Mobiler Speicher”), the other option. This will inevitable delete all data on the SD card, so you will need to make a backup before.
On the specific app: I am using it myself on my FP3+, however, I am running a different operating system and have set up my SD card as “portable storage” right from the start.