example album - "Birth of the Dead" where Disc 1 is "Studio Sides" and 2 is "Live Sides"
Tell Picard to Save all the MB tags, it does so without reporting any errors, but using ID3v3.2 tags, the "Disc Subtitle" doesn't get saved.
Picard also seems to confuse "SetSubtitle" and "DiscSubtitle" between the two different tag versions, just switching the tag version saved from v2.3 to v2.4 fixes the problem without doing anything else.
MP3Tag and other tools allow for TSST tags in ID3v2.3 even though they weren't actually brought in until 2.4.
If the devs aren't willing to go along with a non-standard workaround like this (why not? that's the great thing about "standards" in this area, basically set by pragmatic example by leading-edge vendors) then perhaps consider writing the "Disc Subtitle" to "Set Subtitle" TIT3 in 2.3 tags and leave it to the user to handle copying values around, not a problem with great tagging tools like MP3Tag.
As it stands now one has to switch over to v2.4 tags while gathering MB data with Picard, copy the data over to a custom field, switch back to v2.3 and then copy the data back over, do-able but a major PITA.
Thanks for considering this improvement.