Recently I've been using Mp3tag (v.1.13.5 for Mac OS) to add "GROUPING" tags to some of the albums in my library. This tag is very helpful for organizing tracks in a box set or single-disc reissues with added bonus tracks. Lyrion Music Server detects this tag and groups tracks accordingly when viewing them in its album browser.
With FLAC files, everything works as expected with the GROUPING tag. However, with MP3 or DSF files (both use ID3v2 tags), LMS displays 3 spurious characters before the GROUPING tag text. If I specify the ID3v2 native "GRP1" tag instead of GROUPING, the group is displayed correctly, with no spurious characters.
Here's a screenshot snippet from the LMS page showing the spurious characters before the "Original Album" GROUPING tag:
As an experiment, I used Mp3tag to add both GROUPING and GRP1 tags (tag text: "Test") to three different files: MP3, FLAC, and DSF. Then I viewed the tags in each file using exiftool. For the MP3 and DSF files, exiftool shows three '.' characters at the start of the GROUPING tag (signifying some unknown character), but not the GRP1 tag. For the FLAC file, both tags display correctly. Here's the relevant exiftool output:
FLAC file:
| [adding GROUPING]
| 7) Grouping = Test
| [adding GRP1]
| 8) Grp1 = Test
MP3 file:
| Grouping = ...Test
| Grp1 = Test
DSF file:
| | Grouping = ...Test
| | Grp1 = Test
Since both LMS and exiftool show the spurious characters, I believe Mp3tag is culprit here. I have tried this with multiple samples of each file format, and the behavior is reproducible.
This is only a minor issue, and GRP1 provides a simple workaround for LMS. But if I read the Mp3tag documentation correctly, the mapping between GROUPING and GRP1 should work seamlessly, so I wanted to point this one out.
I purchased the MacOS version of Mp3tag about 5 years ago and I use it regularly (I used the Windows version for several years before that). It has been rock solid and worth every penny/eurocent. Thanks for such a well designed, well supported, and powerful tool.