Fail to see files with "grouping" / which ID3 tag is used?

Issue description:

First, many thanks for a great app! I don’t often buy apps, but this one is well worth the money!

I’ve been looking forward to manage my library with the Grouping tag as I used to in Musicbee.
I have mostly mp3 with ID3v2 tags. I understand there is a confusion in tags used for “Grouping”, so I copied the “TIT1” tag that Musicbee uses into the “GRP1” tag which I understand is the correct tag for grouping. It shows up alright in Navidrome.

However I don’t see any tracks/albums in Symfonium, either via smart playlist or via the Grouping module in the library page.
I’m wondering if maybe I am using an incorrect ID3 field ?

Using the Symfonium latest version on Android 16.
I did clear the tag cache and re-sync. I even completely deleted the provider and re-set it up.
I see a “Custom tag parser” documentation on this support section, but it is not up to date. Googling and ChatGPT are not helping.

Logs:

Upload description: xoub (via in-app “share logs with support” button)

Additional information:

 

 

Reproduction steps:

 
Smart filter with “Grouping” = “is present” : empty playlist
Same with “is missing” : all tracks show up.
ID3 tags inspected with mp3tag and musicbee and navidrome show that some tracks do have a “GRP1” tag filled in (ID3v2.3 for those I checked)
 

Media provider:

Subsonic

Screenshots:

     

As in the many other posts when connected to a server Symfonium does not have access to the tags.

Subsonic does not support grouping tags, there’s some discussions but nothing have moved. (Classical music metadata handling · opensubsonic/open-subsonic-api · Discussion #78 · GitHub)