Amarth reset in 21 Necrotic Wake

We just did 21 NW, had 20+ mins on second boss.
But after our tank pulled Amarth with Fiery Brand he just disappeared. We wasted pride buff and timewarp, which is a huge loss in 21 key and ultimately caused us a deplete and prevent us from getting to 2100 rio.
Here’s the log of events:
https://www.warcraftlogs.com/reports/fby1qz3pa8KnTwHA/#fight=last&type=summary&view=events&hostility=1&source=49.1
As you can see, boss clearly took damage, which is also visible on the replay. But after that he just disappears.
I don’t expect key return or something, but I am legitimately curios why this happened cause I definetely don’t want to see this happening more.
If anyone can provide legitimate explanation, especially from blizzard part, I would be happy to read it.

Ok, first time posting here, post got sent from random alt, so here’s my main just in case.

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