I had the same experience with an UA agent at an outstation. I'm a Canadian permanent resident who flew UA to México. MX waives the visa requirements for Vietnamese citizen with Canadian PR but the agent insisted that I need a visa.

I thought the agent was just illiterate, but now that I checked UA timatic, turns out it will not show the visa exemption unless you select Alien Resident. AC and WJ system is a lot better as they have always issued my boarding passes without a fight.

Actually I had problems with Copa too where they took 30 minutes + to check my visa exemption for Panama, Colombia, and Chile. That even led to me missing a flight, though they did rebook me 7 hours later. I believe Copa uses UA legacy system so no surprises there.

My solution was to simply avoid UA and CM unless there's a screaming deal.