Harvest Boss Immunity Phases Bug

please fix the immunity phase on harvest bosses(not oshabi), its keep hitting and not going to immunity phases when they health reach the threshold, its time wasting and making it not fun, i like doing harvest but its getting on my nerve everytime it just keep running and hitting and not going to immunity phases

Its the yellow one right? God how I hate chasing that mofo down, he just refuses to go into immunity phase.