the last apartment I worked in had mold issues. not caused by
anything other than copper lines in walls that insulation was missing
a 8" piece. after the wall was opened, line insulated mold remediated..
not a big deal since it was caught early enough.

just because I'm curious that way...
I took a look at the heating system & plenum in the upstairs closet.
ended up taking off all the ducts that I could access, mastic sealing the
take offs to the plenums, inner liner to take off and outer insulation jacket
to plenum.
there were 1/2" oversized cuts at every take off. closet was very cool!
all I could think was that when they cut the hole, they didn't cut for the
right size..or maybe misread where to set the hole cutter.
got 4 of 6 done..just cause I couldn't leave it like that. apt renter called
me later the next day to ask what I'd done. said it had never been so cool.

lots of things contribute. blocked/dirty coils, dirty filters, duct/air leakage,
unsealed returns, oversized openings at supply box behind supply grills.
usually apt matinence guys are jacks of all trades, which is what it takes
I guess. but there are times when an educated pro makes all the difference.
just difficult to convince apt managers of this!

best of luck op. maybe you can talk to your manager and get someone
to actually figure out what is going on.