Cannot back up a read_only database with FILESTREAM

I'm facing a strange issue where my backup fails with the message: >Msg 3906, Level 16, State 1, Line 5 >Failed to update database "MyDBB" because the database is read-only. >Msg 3013, Level 16, State 1, Line 5 >BACKUP DATABASE is terminating abnormally. I saw a post talking about in-memory objects causing that issue, but in this case, there is no "memory optimized data". There is a FILESTREAM file but I couldn't find any documentation explaining this behavior. Has anyone faced this issue before? Any way to work around it without setting the DB to READ_WRITE? I think (can't find evidence to prove it) that the backup used to work.