The sudden jump of Android 7 to 10, seems to be an issue for Signal and its backups. I would check for an issue for that on Gitlab, if not found, Report an issue.
Edit: is here already 
S7/S7+ upgrade related issue
Regarding SD card reformat, in another device I have received reminders but I am able to just ignore them.
If you device was originally installed as “extended internal storage”, this is a different case. (I can give a workaround, if required.)
I feel SD cards kept as external storage is best.