I would be furious!! But, not surprised. Why United continues to harass their best customers absolutely stuns me. I, too, and 2 MM, 1K for 20+ years and have not personally experienced this, but I would send this message to the 1K Voice. This is nothing short of harassing the best customers. It is not about TSA requirements.
I was recently harassed by an EWR gate agent as well. A not even remotely too large bag ... he made me put it in the sizer ... of course it fit just fine. Boarding took forever and the flight left late. If GAs cannot tell oversized bags from legit ones they need to stop making a fuss. I 1000% agree to stop letting people on board w/ too many or oversized bags ... but if this leads to a harassment of all of their (premium) customers and delayed flights it is not helpful.