Don't complain about the ua.com, any booking channel of the OP's itinerary suffers from this.

OP is seeing a lower CUN-ROC oneway (LXA10 fare basis) than is offered when pricing the roundtrip (T0MX1F fare basis). I'm not sure why (point of sale restrictions? combinability?) but it's not the fault of da bomb.

(Your specific experience on .bomb on a different itinerary may well be a different issue).