That was an interesting puzzle. I think this might work:
The 'Missing' tag should then display the track numbers that are missing from an album.
To catch if an album has tracks missing also for the above cases, you will need to combine this with MusicBee's built-in 'Album Complete?' feature.
Also, it doesn't seem to update real-time, so you might need a restart of MB to update the 'Missing' output.
$IsNull(<Album>,,$IsNull(<Track#>,,$Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Replace($Pad(<Track#>,2),01,3),02,30),03,300),04,3000),05,30000),06,300000),07,3000000),08,30000000),09,300000000),10,1000000000),11,10000000000),12,100000000000),13,1000000000000),14,10000000000000),15,100000000000000),16,1000000000000000),17,10000000000000000),18,100000000000000000),19,1000000000000000000)))
$IsNull(<Album>,,$If($Right($Sum(<Numerals>,<Album>),1)=0,"1 ",)$If($Left($Right($Sum(<Numerals>,<Album>),2),1)=0,"2 ",)$If($Left($Right($Sum(<Numerals>,<Album>),3),1)=0,"3 ",)$If($Left($Right($Sum(<Numerals>,<Album>),4),1)=0,"4 ",)$If($Left($Right($Sum(<Numerals>,<Album>),5),1)=0,"5 ",)$If($Left($Right($Sum(<Numerals>,<Album>),6),1)=0,"6 ",)$If($Left($Right($Sum(<Numerals>,<Album>),7),1)=0,"7 ",)$If($Left($Right($Sum(<Numerals>,<Album>),8),1)=0,"8 ",)$If($Left($Right($Sum(<Numerals>,<Album>),9),1)=0,"9 ",)$If($Left($Right($Sum(<Numerals>,<Album>),10),1)=0,"10 ",)$If($Left($Right($Sum(<Numerals>,<Album>),11),1)=0,"11 ",)$If($Left($Right($Sum(<Numerals>,<Album>),12),1)=0,"12 ",)$If($Left($Right($Sum(<Numerals>,<Album>),13),1)=0,"13 ",)$If($Left($Right($Sum(<Numerals>,<Album>),14),1)=0,"14 ",)$If($Left($Right($Sum(<Numerals>,<Album>),15),1)=0,"15 ",)$If($Left($Right($Sum(<Numerals>,<Album>),16),1)=0,"16 ",)$If($Left($Right($Sum(<Numerals>,<Album>),17),1)=0,"17 ",)$If($Left($Right($Sum(<Numerals>,<Album>),18),1)=0,"18 ",)$If($Left($Right($Sum(<Numerals>,<Album>),19),1)=0,"19 ",))