I use (and have always used) %DISC% to hold disc number information. This has always worked fine, and continues to work fine for the majority of my files (FLAC).
But now I find that on MP3+Apev2 files, any attempt to set %DISC% sets %DISCNUMBER% instead (seen in 0.9.3.1, still present in 0.9.4.1). I also find no option to disable this behaviour. Why is foobar deciding what I can and can't store in tags?
Splitting nn/NN style tracknumbers into two tags was somewhat surprising, but at least understandable (even though it broke my playlist script); but here I don't see any reason why foobar would prevent me from setting a %DISC% tag to any value. There certainly isn't anything in the APEv2 spec requiring this behaviour...

Preferences > Advanced > Standard Fields

i personally prefer DISCNUMBER but you can do w/e you want.

I have the "standard fields" setting set to use DISC as "Disc Number". But when I set that field for an MP3+APEv2 file, it is not set, and a <DISCNUMBER> tag appears instead with the value I entered. So it seems like DISCNUMBER is hardwired in some cases. Maybe DISCNUMBER is the more "standard" field to use, but I should be able to use what I like (not exactly inclined to retag 18000 files for this).

Bump due to no suitable answer so far.

Maybe there isn't one. Bumping won't change that.