This is a rather odd issue that has apparently been around for a long time and never resolved. In searching the Forum for answers, I was surprised to find myself trying to help someone else (
Why does MC14 Playing Now mix track order and filename order playback??). Now, more than a year later, I have the same problem. I suspect the condition has been there all along, but I haven't noticed it until now...
Today I sent an album to PN and was surprised to see that it turned up there in alphabetical order by track name. It was sent by double-clicking the album in an Album pane of a Panes View with
Sort inside groups by set to Default (which I understand to be [Disc #] - [Track #]). The tracks appear in the correct order in the Panes View, [Disc #] is empty and
Shuffle is off. Other albums work fine. Not surprisingly, this album does exactly the same thing when sent to a playlist. Other albums work fine. I tested 100 albums and found four behaving this way. Two of the four end up in [Name] order in PN. One had only one track out of order. The fourth seemed random. The first two also had the same [Date Imported]. Of the eight imported that day (in December, by MC15), four have this issue.
The workaround is to make a [Disc #] - [Track #] sort preset, and then use that instead of the Default. I suppose that's not so bad, but it's still disconcerting that something so strange and random is going on. Also, only a small portion of albums being affected suggests something is "wrong" with those records—which I have no idea how to fix.