I had new tires put on my car last fall, never with any issues with the TPMS. Interestingly, was at the dealer for service (where I presume they top off the tires) and about 2 weeks later my TPMS sensor went on. Found Lt rear down to 18 lbs. I topped it off and again this week light went on and tire at 20 lbs. Took it to my tire place, a reputable family owned local business- NOT a chain.
They noted a small leak from the TPMS valve and shows me the pin was not sitting correctly. They tightened it best they could and lets see what happens.
I would be hard pressed to accuse the dealer of using a gorilla hose to top off my tires.

Question then is, if my valve continues to leak, I see that there are "service kits" available. Is this something I can do or does the tire have to be dismounted?

When the tires were replaced, no servicing was done to any of the valves.