Meant to put:

Didn't really think too much of it until it had maybe 3 weeks left...

This pic is what it started out like in early flower, on the right (3-4yrs stale no name organic soil) this time and the other (fresh FF soil) on the left. About a week after that pic, the leaves started getting a rust looking layer forming in the middle & starting with the oldest fan leaves. I flushed like you did and it seemed to stop the progression for a few days, then it came back even harder with other nutrient deficiencies and nute-burn looking symptoms. I looked a little for solutions this late in flower but ended up just continuing as if everything were normal. I tried smoking a bowl from that plant and it was very funky tasting and generally not so strong so it's in with my growing pile of trim that I'm saving for some hash. I think there was about 3-4 grams of basically sticks with a few pairs of preflowers runing down them. Even though it was a dwarf autoflower that's still extremely pitiful: I almost didn't even save it.