I had similar frustrations back in October 2020, and eventually put it down to a problem in Android. I fixed the problem by initializing the SD card for mobile storage, as suggested by other posters.
I just tried connecting my FP3 to my Debian home computer (Debian 9 and Gnome 3.22.2).
The FP3 made an alert noise, asked me how to connect; I chose “File Transfer”.
I opened Nautilus (file system explorer) on the computer, and saw in the left hand panel a device “FP3”.
I clicked on that device, and saw two entries: a disc icon labelled “Internal shared storage” and an SD card icon labelled “Samsung SD card”.
I clicked on the SD card, was able to navigate through the directories to get to photos taken using OpenCamera… everything seems to work normally.