inconsistency with multi-disc

[mweather]

I have a problem with the way multi-disc sets are displayed. The discs are differentiated as 1/3, 2/3, etc. Sometimes the contents are displayed as the manual says, with the subsequent disc in a title being renumbered after the previous discs. At other times, the tracks are simply ordered by track numbers (1 followed by 1, then 2 and 2, etc.). This makes nonsense of works like operas. I can't find any difference between the sets that work and those that don't. Please help.