common
problem in all air-conditioning not just cars, its fluff and particles that stick to the inside air-conditioning coil (behind the glove box in most VAG cars) bit like tumble dryer fluff, it gets wet with condensation when the ac is running and moulds, the smell goes away when the coil drys and comes back again when it gets wet.
In normal AC you would use a coil cleaning solution, a small high pressure spray and a paint brush to remove it,
problem solved.
In cars its a little bit more difficult because getting to the coil to clean it is hard, comma works by killing the bacteria living in the fluff but doesn’t remove the fluff so it comes back sooner or later.
I’ve used comma on my cupra last summer, worked fine but the smell came back a few months later, now I carry an aerosol of detol anti bacterial surface spray (kills 99.9% of all bacteria don’t you know) and i periodically put the AC on recirculation and spray upwards from the passenger foot well (air onto the coil when in recirculation) for 10-15 secs.
Works for me, and working within air-conditioning sector for 10 years i know its what lazy engineers do to clean ( without getting their hands dirty) whole offices of indoor units!!
hope this is helpful