This bug happened on Season of Discovery,
Not sure how to reproduce it, nothing out of the ordinary really happened during the fight.
Xoory-Crusader Strike 8;03PM PT, here is the log for the attempt: sod.warcraftlogs[dot]com/reports/Hyh68GmpazXvf1KZ?fight=63&type=damage-done
My guild got Kel’Thuzad to 1 health point during Hardmode 4 and he didn’t “die” to begin the transition to the last phase where he reanimates and resumes combat. The only weird thing that happened during the attempt was that he only did 1 MC the whole fight prior to getting him to 1hp with it just being off cd/queued up for the rest of the fight until he did a 2nd one after 3 minutes.