I had a single experience in this area and had the SAME issue as the OP.

The phone agent had the last digit of the fare code wrong - instead of Y, he used N - to make things worse, the fares are IDENTICAL for both. The fare basis for both is the same, a V class. Fare codes only differ at the last digit - N versus Y.

I did not catch the error when ticketed it at MIA where the agents would NOT make reservation at all and directed me to call reservation to get a PNR, claiming they only do exchange / issue new tickets at that station.

Long story short, I found out the error at home 2 1/2 hours later. Immediately called UA reservation. It took 3 phone calls - the first one got disconnected while the agent put me on hold, the second one the agent was so incompetent I hang up, the third one after the agent went back and forth to consult with her supervisor 3 times I demanded to talk to the supervisor - a supervisor at one of the call centers at Philippines exchanged the N ticket with the Y ticket "because I booked this less than 3 hours ago."