[DaveBlake]

Check your tagging

See Foobar image in the first message

"Disk Number" on that screen shot is what Foobar labels that tag. It shows that Foobar has read some tag it considers as a disc number, but it does not show me what the Vorbis tag was actually called. But anyway it would seem the issue is cuesheet related.

Do your cue sheets have lines "REM DISCNUMBER" ? That is where Kodi looks for the disc number

No.
I've never see in a .cue file the DISCNUMBER tag.

Anyway.. same audio library in XBMC
Work fine.

Well whatever old versions did, and sorry I don't have time to dig that far back into the code, Kodi now processes the cue sheet to get both track and disc number. I have no idea if that is a common standard or not, but it is what it does. It was added in Gotham, but I am not sure that before that disc number from the tag was being used in Eden or Frodo.

You could add "REM DISCNUMBER" with a value to the cuesheets to confirm that solves the problem.