The album tracks are sorted by disc and track number. What you described can happen if there’s no disc tag on the tracks, or if you disabled the disc tag in MAFA or in your MPD configuration file in the value of metadata_to_use.
If the disc tag exists and is not disabled each track item displays the disc and track number like this: “Disc 1 - #1”. Otherwise only the track number is displayed.
Another possibility is that MAFA can’t parse the value of the disc tag as integer.
Can you check what is the exact value of the disc tag? Is it a number or does it contain other characters.