How to reproduce:
With these items: 
Reddit Link: https://www.reddit.com/r/Rivenmains/comments/637gid/can_someone_explain_how_karma_lived/
Zielmann2 votesMANTRA - RENEWAL: Focused Resolve heals Karma for 20% (+ 1% per 100 AP) missing health upon being cast and once more upon rooting its target, with the root duration increased.
Basically, she healed from the mantra W almost exactly as much health as you dealt damage.
Friendly Ram1 votesyep so few people realize karma has a heal.
Gets Colder1 votesSo she healed for 689 damage assuming that it's doing max damage which it I believe should with that amount of HP? Even with the armor I don't see how that's possible with that amount of AP even though she gets a flat 20% missing health increase which should leave her at 293.4 HP which is about 19% of her actual Health.
Zielmann2 votesHave some math
Karma has 50 armor, your ult does 689 damage. 50 armor means Karma will take about 66% of that damage, which equals 459 damage from the ult.
Karma has 1572 total HP. She uses the mantra W at about 105 HP, so she's missing 1467 HP. She also has about 50 AP, so her 20% missing health heal will scale up to about 20.5%. That's a base heal of about 301 HP.
Now, factor in bonus heals/shields. With the two bonus heal/shield masteries, that heal will bump up to about 358 HP healed. Add to that the 105 HP she was at when she used it, and she's at 463 HP. But your ult only does 459, thus she lives at about 4 HP.
And this doesn't factor in any additional bonus healing/shielding she has from items, since I can't see what she has. She likely at least has another 10% bonus (redemption), possibly 20% if she rushed Crucible. Even if it's just the 10% bonus, that's a 394 HP heal, added to her 105 HP she had to start. That puts her at 499 HP. If she was at 20% bonus healing/shielding from items, she'd end up with 534 HP after the heal. That's likely enough wiggle room to counter any armor pen or %bonus damage you had at the time.
Gets Colder1 votesI love you! I can sleep at night! But the items are shown at the bottom in the video and I added them in the description if you want to know. But you are right.
Zielmann1 votesAh, so she only had the forbidden idol, which is only 8% bonus from her items. But that still puts her at a final HP of about 492. She escaped at 7 HP, so your ult still did 485 damage to her.
It just looks odd in the moment cause everything happened at once, and it's a larger heal than you'd expect!