Almost four months later, and it's still very buggy. I think I know what's wrong, though. The drop ships count against the timer whether or not they're inside the drop zone, but sometimes their pathfinding breaks, and they get stuck on some terrain on the plains (similar to how the smaller fighter craft sometimes get stuck on the ground), meaning they never make it to the drop zone, but still tank the timer. This can happen with multiple drop ships, which is why sometimes the counter drops like a rock, and sometimes it just drops a little faster than it should. Simple fix: make sure the drop ships only register with the timer when inside the drop zone. If some minimum number of enemies aren't inside the drop zone within a certain window of time (because they got hung up on terrain), send them in directly with drop pods, instead. I know pathfinding bugs can sometimes be the worst to fix, so this should be a simpler solution.