Happened to me before but only after an agent at customer service called in to try a change (I had a companion that against my wishes got a split PNR). They called into UA ticketing and the operator put a note on my record to not allow any more changes. Never happened to me using the app or online.
My only advice is to never talk to a person about changes unless its absolutely needed, e.g. return flight somehow ended up before departure flight. If then call MPSC and not someone at the airport unless its a gate agent. I learned that trip that gate agents provide the best customer service on the ground.
I also felt the same way about UA as you but I got over it. This was a one time thing on one reservation and they will probably never do it again on another reservation.