[Update]: The latest update has now corrected this problem, as well as added a whole bunch of other changes and content
Out of all the heroes in Overwatch, both new and old, none have been as controversial as Brigitte. She not only ushered in the highly annoying triple tank and triple support meta, but her initial incarnation was so ridiculously powerful that she received nearly a dozen nerfs in a row without ever leaving the meta.
With that in mind, you might be interested to hear that apparently a part of her power is due to a rather devious little bug. Instead of simply giving Brigitte and her allies additional armor, it would appear that her ultimate Rally constantly gives more and more layers of armor, each of which stacks to further reduce the damage of incoming attacks. Once all fifteen armor layers are fully stacked, this bug can reduce incoming damage by 45% more than your standard armor, which would explain why Brigitte feels downright invincible during Rally!
Since this is a bit difficult to explain through words alone, I would highly recommend you read ClumsyLi's analysis on Reddit as he's the one that first discovered this problem. Once you're done with that, you should also check out DarkyPaky's brief video example as it pretty much confirms the bug and shows that the numbers are the same in both theory and practice.
Not everything is bad news, however, as Blizzard's Bill Warnecke has already confirmed that the hero design team will be looking into this further. Whether we'll get any sort of changes in the near future, that still remains to be seen, though given the complexity of the problem I think we're in for a little bit of a wait.
That said, I'm not exactly sure what the correct course of action is here. Brigitte is already mediocre outside of triple tank and triple support compositions, but absolutely nightmarish to deal with as a part of those compositions. Nerfing her ultimate would make triple tank and triple support slightly weaker, but it would also further cement her as a sub-optimal hero for ranked play. Honestly, I think at this point Blizzard might as well cut their losses and just do a Symmetra style rework for Brigitte. Everything else seems like it'll just result in more problems than solutions.
Whatever the case may be, I'll make sure to let you know once Blizzard announces their plans for Brigitte's future. Until then, you might want to check out the latest short story starring Baptiste!