Using Wavpack files (lossless mode), if with the masstagger I do:
Format "TRACKNUMBER" using "$num(%tracknumber%,2)"
nothing happens to the first 9 tracks of an album (tracknumbers should become 01, 02, 03 ... 09 but they remain as 1, 2, 3 ... 9).

I noticed that I don't get that issue with mp3 files.

Intended behaviour or something?

Actually this happens to mpc files too (probably every format using apev2?).

So is this a feature (limitation) of apev2 specifications?

BTW using/creating DISCNUMBER with foobar2000 (apev2 tags) is seen as DISC by other programs.