Is it a bug ? no minions during fight with Harbinger of hatred

On the fight with the harbinger of mephisto , all my minions disseapeared and could not be reinvoked/summoned (though there were bodies…it just sprung the skeleton priest) . luckily i managed to blood wave lance him but for a summoner build it’s kinda louzy to loose 98% of your fire power on a boss fight. or was it just a bug that made them invisible ?