If it's been ticketed as V on UA, that is the lowest of the low (almost) and is an L fare. You have been very lucky (after V fare upgrade policy came into effect), to get upgraded on V fares on UA tickets.

You can easily do a fare check to confirm on Expedia. Just look up the UA and AC V and L fares for the same city pair. You should find V on UA = L on AC (or some other non-upgradeable fare).

The UA and AC fare (and other carrier's) systems are not compatible and you should not expect to be upgraded on UA V fares. If you do, consider it an added bonus. In addition, getting upgraded on UA metal (not this trip) is tougher than AC metal.
Prior to V fare policy, this really didn't matter.

Yes there is a fare equivalent on UA for every AC fare and the letter codes are different. Confusing yes but AC was right in both cases. AC did you a favor by upgrading you on their metal on a UA V fare and just enforced the rules on the return flight. Be happy!