You can only mix down per part (which works fine btw
) , ie: if I have a composition with say 5 parts , each of which is like 32 bars long but i only want say 2 bars, Luna would still be mixing down the whole song, regardless of where my loop region is, creating a file with huge recundant bits when all I wanted was 2 bars of peaktime 
.Splitting your parts up is a great workaround, but still...
It's not a bug per se, more of a cumbersome fact.
Marco