Let's say I have an album that is comprised of 2 discs and 20 tracks (10 tracks per disc). I have only ripped one disc and added it to MusicBee. MusicBee sees that I have all the tracks on the first disc (10 tracks) and is unaware of the tracktotal for any additional discs so it reports the album as complete.
Are you sure about this?
From my testing, for an album with a missing disc, as long as the <Disc Count> field is set to the correct number of expected discs, then the <Album Complete?> field will report "?" (and not "Y"):
I can't claim to know the thinking behind this - maybe reporting that it is 'unsure whether the album is complete as there appears to be a disc missing'.