This issue is related with this topic, closed now:
In the latest version it works good, plain view + drag&drop, (multiple) folders + drag&drop, F4 multiple files in plain view, late thanks for the fix - just this one still doesn't work:
Drag&drop folder with subfolder from FreeCommander (using %ActivSel% parameter) to mp3tag.
To reproduce, create folder A, create subfolder B in folder A, place some mp3 files in folder B, now drag&drop folder A to mp3tag - it doesn't work. If you drag&drop folder B to mp3tag directly, it works good.
So, if you move many folders, like 30 and 3 have some subfolders, you can miss all files in these subfolders(!).
Workaround, use a plain view (Ctrl+B) to display all files, select all files, focus any file and press F4 or drag and drop to mp3tag. (You can't select all and focus [..] item and F4, this way Notepad is opened)
Using mp3tag 2.94, FreeCommander 794 64-bit, Windows 10 64-bit 1809.