Didnt you just have a low coolant issue?
You probably filled it up again, but some air was still trapped in the system and that air is now trapped in the heater core.
Maybe do a coolant bleeding procedure that maybe gets the air out?
thats just my guess... would not hurt to try though?
guide how to do it(its quite simple):
https://blog.fcpeuro.com/how-to-use-...ding-procedure
__________________
2009 35i DCT Havana brown with ivory white interior. Mods: Cobb stage 1+FMIC, Wagner EVO1 FMIC, BMS DCI, KW V3 suspension, M3 front control arms, Hardrace RTAB, Michelin Pilot Sport 4S tires.