Had this happen a couple times as well.

Starting to think it might be like Ifrit EX's enrage during the last nail phase.

Possibly DPS pushing too much before the 2nd tidal wave results in a phase change and the shield not proccing and getting hit for the same amount of damage 30k~ as the hard enrage after the third set of warrior adds spawn.

We tried slowing DPS at the part right after the 2nd spumes are killed until the button popped. Didn't happen again.

Seems like it happens more often when the DPS is high enough to skip dive phases or you clean up the 2nd spumes quick using Meteor.