Poweramp doesn't seem to correctly read the DISC field in any format which uses APEv2 (WavPack, Musepack, etc) and will report the disc number as 1 no matter what number is actually entered in the field.
It should be easy to reproduce by converting a flac to a musepack or wavepack in Foobar2000.
The only workaround is to brute force a DISCNUMBER field, although DISC seems to be correct fieldname at least going by this table: https://picard-docs.musicbrainz.org/downloads/MusicBrainz_Picard_Tag_Map.html
On a related note: entering total tracks into a DISC, Track or Discnumber field by using a slash also breaks the reading of the tags. For example 14/14 to specify track 14 of 14 total tracks.