When did they become Complexity-Limit and why?
I am certain blizzard wanted more from the mythic-only phase, but eventually they noticed the fight was getting insanely long. They cut some things and the boss still took 13mins to die with most guilds killing it the last second (limit killed it after enrage). I don’t think blizzard cut things because they were buggy. They just cut it at the last minute so it looks kinda messy after they overtuned the encounter for top guilds.
They can disbelieve Ion all they want, the time they stayed in the Heart Chamber revealed that N'zoth's enrage timer was 13 minutes. There's no way there was ever enough time for a second mythic phase.