I’ve played Container City a lot, and never seen this bug before.
I was on defense, and somehow the maintenance bot advanced right through the gate without the attackers actually detonating a charge, like some sort of ghost bot. The mission did not change until the attackers actually blew up the gate, but when that happened, the maintenance bot was already well past the gate.
Maybe there’s no collision detection between the maintenance bot and the gate, and somehow the bot’s trigger to move forward was activated without the gate actually being taken down.