By pure coincidence, I think I managed to discover something. As I said, sometimes the unit works like they should (apart from the bow sounds), sometimes they don't. Turns out, they start acting up whenever I enable skirmish mode. I tried it in a battle earlier. Enabled skirmish mode, sporadic fire. Disabled it, entire rank fired. Enabled it again, problem came back, and so on. I have no idea why that is, though.