This is the place where displaying percentages falls down, unfortunately. We all know that we're rolling dice, and that rolling a 1 means a miss, regardless... so your chance to hit is never 100%, no matter how stacked the odds are in your favour.
The reason you can sometimes see 100% displayed is that your technical percent chance to hit when you need only to "not roll a 1", and you have advantage, is over 99% (There are 400 possible results in an advantage roll, and only one of them results in amiss - that's a .25% chance of getting that one. This means you have a 99.75% chance to hit). It's not 100%; you *can* still roll that double natural 1... but the game's displayed percentage rounds to whole figures, and fails to properly communicate this. I'm not a fan of the percentage display - it feels misleading at times, and I think it's a partial contributor to people feeling and remembering their strings of misses more. By abstracting the result from the dice, it's much easier for our brains to end up feeling like it must have been unfair.
In this case, it's likely that even a 2+ you attack modifier was enough to hit, so with advantage, you had that 1 in 400 chance of missing (double natural 1); you got it, and so it looks and feels unfair and wrong, because the game was reporting 100%.
That said....
You have a screen shot before and after, which tells me you were anticipating this to some extent. If this has been happening often enough for you to be prepared to screen shot it when it did, then that feels like something more serious may be wrong.
