I've seen something similar happen a couple of times where I can't select a stack when a nearby stack is alraedy selected. What I usually do is scroll out, select another unit and then come back to select the one I want. Maybe this isn't even the same issue, since I've never encountered anything major beyond that (let alone over a number of turns).
This doesn't help because I can't activate any of my other units. Well that's not entirely true, I can double click the cannon to select the other units on the same plot with it.
Thanks for confirming there is an issue