Why not ask P2 to flush the A/C? Its a rather simple process nowadays with machine.

Personally I had one instance like this in the past. I drove a Sonata back then, went to top up gas. When I first starting to drive, it was cool and then after a couple minutes, it turns into a heater. Went back to the shop, they released the gas, suck all the oil, reoil and then re gas. Everything went back to normal since. If P2 cant fix it, tell them you will goto outside shop to fix but they have to pay the cost.

Probably the gas pressure too high since brand new car.