It is a mystery to me. I use a lot of ACC, and identical tubes of the thin fast stuff will respectively last 1) one day 2) one week 3) one month or 4) 6 months treated exactly the same. I too religiously keep them upright, and when used, I spread droplets onto small folded squares of aluminum foil (I make a dozen at a time).
I will second the suggestion that you buy only the best branded thin stuff. You can purchase imported tubes for as little as @$0.10, and while some tubes do OK all around, others are best propelled directly to the trash- and you never know which is which until you open them up and attempt to use them.
Now, as we speak, and right at hand is one of the hobby shop brands of thick CA (Zap-a-Gap Medium) in a bottle. It is an excellent adhesive, and is a first line tool on my bench. It has now lasted three years, and just a few seconds ago, it flowed right out of the bottle to repair a locomotive brake hanger. The bottle top is encrusted with hardened stuff, and every once in a while I go at it with hammer and tong to clear the stuff off so that the lids continue to fit tight. Every day I expect to find everything hardened, but as yet there is a quite visible and reassuring meniscus of fluid in the bottle that gently rocks back and forth when the bottle is flicked.
Now, will the next bottle last as well?
Denny S. Anspach MD