When those tags are not present, everything works fine. When they are , i.e. I reopen album in Picard later to fix some tags and I get above tags doubled as lower case along with original case:
Picard alone added the lower case lettered tags which are not present in the metadata. Here is the screenshot of the Foobar2k song properties:
Yeah, that fixes it, tags are again in the result uppercase and there are no lowercase variants.
This weird behavior should be fixed anyway, unless I am mistaken about something crucial. I see now that Foobar converts lowercase tag names to uppercase on save, so maybe its that custom tags MUST be uppercase. But even so, behavior is IMO not correct - I shouldn’t get lowercased tags then anyway.
Picard scripting is case-sensitive, and in general, you have to handle MP3 differently from M4A differently from FLAC/Vorbis, which requires lots of branches in your tagger scripts.