I have had a small amount of stay-flo mix go bad. I hadn't used it in more than a month, and it was stored in my laundry room on the shelf. There was a little mold inside the bottle. I dumped it, cleaned the bottle, and made more. It's really cheap to make, so if I lose a little, I'm still saving $piles$ over buying other bottles of spray starch. Probably would be a good idea to store it in the refrigerator.
I also found that if I haven't shaken the bottle in a while, it is more likely to flake, and the reason for that is that the starch and water have separated a little. I've learned to pick up the bottle and shake a little as I get the bottle situated in my hand. If it hasn't been used for a day or two, I shake a little more at first. And the flakes don't hurt anyone anyhow. Ignore them and they will go away.