Hello we exchange by email month ago about the exact same problem. And I have also the same configuration as my folder (called lgg4) is a ds cloud folder (Synology application). It was working since years without problem. But recently this year all subfolder of lgg4 folder display no more photo in f-stop.
(But photo are still there).

Particularity of this lgg4 folder is that it is located in sdcard in android/data/com.synology.dscloud/ folder and there is a .nomedia file in android/data folder.

Lgg4 is manually added and display photo. But before new version of f-stop, subfolder were also included.

If I add manually a subfolder in f-stop the content is displayed. But I can’t add manually more than 50 folders !
I played month ago with media scanning option and it resolve my problem but now it no more works.

Do you have a workaround to resolve my problem ? Ps : I sent you email this week about my problem