You could try a little
oil or grease on the threads. Possibly liquid teflon pipe goop. I use yellow teflon tape frequently.
I also use brass fittings into aluminum. AN fittings are often aluminum and I'm not 100% sure, but I think they may not be NPT threads. So you may have had threads starting to interfere with each other.
Maybe Google "galling" where the friction of tighening actually welds the metal together and see what remedies there are.
In your situation, putting the whole thing in the deep freeze for a while might have helped as the fitting should shrink more than the housing. Pouring hot water on the housing after the deep-freeze thing might hep even more. But nothing will help if it did gall.
WD40 might also have helped, but regular penetrating
oil doesn't seem to do much for aluminum corrosion.
OOPS...I just reread and see you used an NPT-AN adapter. er.....or maybe an AN-NPT adapter?