First noticed by Overwatch Central, Orisa's ultimate Supercharger seems to charge up while the bongo is out and is active. This means that Orisa can stack ultimates, as she's able to keep dealing and blocking damage to build more percentage. She also gains the percentage faster than normal since her damage is boosted.
Characters that have a damage boosting mechanic are typically not allowed to build their ultimates while it's active, like Ana and Symmetra. This is broken for so many reasons, and we'd expect Blizzard to hotfix the issue promptly. We wouldn't be surprised if Orisa was disabled from play fairly quickly to address these concerns.
Blizzard has said they are looking into the bug immediately preceding this article going to press, so expect a response from the Overwatch dev team within the next short while.
Overwatch Patch 1.12 contained a brand new map chock full of lore references, balance changes (which some are calling the death of Roadhog's hook-shot combo), and even an undocumented change that broke certain aimbot programs.