I died inside Kindred's ult

Watching in .25 speed it looks like you got hit right as you flashed.

Unfortunately due to how computers work things like these are simply unavoidable.

Depending on the order of operations in the code there are a number of ticks you could have died during. At max: Death check, Damage Taken, Kindred Ult Update, positional update, flash input. Which leaves you with 5 frames you could have died during, or 1 frame if the checks are done in the opposite order which I highly doubt. At a guess I would say it takes the game 2-3 ticks to apply everything in that situation correctly, with most of it being done in the first frame.

The point is that Computers can only do 1 thing at a time, they just do each one of those things very very quickly. It does mean though that the game probably registered your flash, updated your position, then you got hit by the damage, it exceeded your health and you died, then (if you hadn't) the game at some point would have gotten to the Kindred ult in existence, checked to see if anyone was in it, and then applied the Ult Buff to everyone which would have prevented your from dying.

While these issues are Technically solvable, they would require so much redundant programming that it would have a drastic impact on the game's performance, not to mention how difficult it would be to create such a thing without introducing numerous bugs. League servers update 30 times a second so things being 2-3 ticks off isn't actually noticeable, we really only notice them happening due to Leagues visuals (The flash particle effect, your body falling down in the Kindred ult)

For reference I play at around 50 ping (.05 seconds), which is a barely noticable delay (and I was only able to notice after the server move since I was at 24 before), and equates to me playing a little less than 2 ticks behind the servers.

TLDR:

Yes it isn't supposed to happen, and thus is a bug, however it is due to a computer limitation and can be found in any and every program on your computer, it is just all but impossible to notice without visual guides.