That was actually intentional although the behavior is definitely different than standard UA. Here's the description in the latest version of the docs:
A word about Quest Stages: OAUA handles these slightly differently than standard UA, in a way that is easier and less prone to game-breaking Quest Stage bugs for newer designers. In FRUA, the Quest Stage event will only fire if the Quest Stage is equal to a specified number; in OAUA, the Quest Stage event will fire if the Quest Stage is equal to or greater than the specified number. What they have in common is that the Quest Stage event will not fire if the Quest Stage is less than the specified number. The OAUA method opens up some new possibilities for designers, such as the ability to track whether a certain event has happened at any point in the game, or to easily create non-linear Quests, or even to calculate whether the player is less than, equal to or greater than a certain Quest Stage. The potential drawback with this is if you set your Quest Stage events to happen more than Only Once, there is the potential for the player to continue to advance the Quest Stages by repeatedly triggering the same event. I recommend that designers who regularly use Quest Stage events be aware of this difference when planning your design, and playtest to make sure it works as intended.