Forensic possibilities:
1. Just dumb luck. Nothing much you can do.
2. Something in tire. If you can't find it, maybe swap front and rear tires to see if your problem follows the tire to the front.
3. Something in rim. Usually this would be a rim tape problem or a sharp burr in the aluminum. A close visual inspection and tactile inspection of the whole inside surface should find it.
4. Problem with mounting. Pinched tube under the bead, very unlikely your mechanic would make this mistake even once, let alone multiple times. Also this problem usually presents itself in an obvious way.