According to our programmers, such problem 1) hasn't been reported and 2) it doesn't exist.
Your mercs was alive for sure ? and also, are you sure they already didn't have a potion on them.?
Sometimes, because of lag, after a performed battle( using your mercs in your case), you don't see the potion, so you have to fight one more time, in order to put a new one.
Imo, if you put a potion for 5 battles, count them, if you can't use a new potion after these 5 battles, then we certainly have a prob. Although, I tested it, and it was ok..