I tested a single rack with discogs release 3351712
In MP3Tags V2.86 the track number as displayed in the Adjust tag information screen is 1
In MP3Tags V2.87a the track number as displayed in the Adjust tag information screen is 1
In In MP3Tags V2.87c the track number as displayed in the Adjust tag information screen is A1!
The attached demonstrates the source of the problem I have. @Florian may have introduced this change for other reasons, but it's having unintended consequences.