I’ve got an update on this:
I solved this by accident (I think). I was wondering why only five of my 29 Ampache catalogs were being shown in the filter menu. So I had a look at the database table to check if I could find a difference between the catalogs being recognized and those not being shown.
There was one: The catalogs that are visible in the filter menu had a value ‘music’ in the column ‘gather_types’. So I manually added ‘music’ to all catalog rows and synchronized the Symfonium database. That led to all catalogs being visible in the filter menu. I then switched to Ampache 6.3.0, did the same, it worked and also the same number of artists, albums and titles are now being discovered by Symfonium.
I have no clue why this wasn’t a problem before but am glad that I figured it out.
Maybe the ‘gather_types’ column was added after my initial Ampache setup. I have been using the same database for at least ten years and maybe these old catalog rows were never correctly updated once the ‘gather_types’ column was introduced.