I have heard of cronic dust from cellulose leaking into houses. Maybe
your AC system is somehow pulling down attic air from a poor instal or
not being sealed up right. Supply and returns are in the attic covered
by insilation? There is a way to test ducts for air tightness or just
seal them with Duct Mastic and be sure they are screwed together right.
I dought your filter is good , get a large media like Air Bear, April
Air is crappy since the case wont seal to the insert well. Maybe you
can get a Media replacement to slide in your case and get it tight by
using foam window insulation that comes in rolls. Try extra room
airfilters and maybe a better vac or the small micron bags.