The mounts all have a rubber core in them I'm sure, for vibration and noise resistant. It could be possible to have a defective one that had a flaw in the center rubber that didn't tear all the way or most of the way until it got some miles on it and torque applied a few times. Not out of the range of possibilities. I have purchased many brand new parts in my day that were bad right out of the box. It's kind of a fluke, but does happen occasionally. At least it's a minor issue (cost and fix), so I hope it's that or something even simpler.
As I said, I'm just making a guess.