What I found is not so much a bug as a flawed gameplay mechanic. When Goblin Archers spawn early in the game, they tend to use ranged attacks on any unit that is next to them. This works well. What doesn't work is how the AI handles the situation when their unit is attacked. They don't run away from the archer - they attempt to fortify until healed. The problem is, under constant archer barrage, the unit never fully heals, and thus stays under archer attack indefinitely. This results in savage archers having Combat V, Drill IV, Archery III, and a myriad of other promotions around turn 100. These archers then fortify and never move, and when your civilization expands around them, they're a constant nuisance you can't get rid of because they're so powerful your units can't even damage them, let alone kill them.
Do you know how hard it is to kill a unit armed with poison, +100% strength, 3-7 first strikes, and able to take half the life of any unit of yours that comes near?