So, the siege engine decay rate and repairing efficiency went up. I thought this would make repairing your catapult a good idea, but in practice, the extremely fast decay rate kicks in very soon after you stop operating the engine, and your repair work can't keep up.
I think it's a bit silly that the engine decays more when you are actively trying to fix it than when you are using it. Maybe applying the repair should affect the timer a bit. Or maybe my tank just needs a lesson in operating his tools.