The Barber Malignant Heart doesn’t stop Lucky Hit abilities “When you deal damage” and “On-Hit“, 
Hewed Flesh and 
Drain Vitality. As a result, any player’s Barber activation will steal damage from other players. You will notice the damage activation from the explosion but others will not see that.
Mobs with relatively high health, like world bosses, allow you to stack multiple millions of damage during the activation phase, meaning this can help build stack tons of damage on Echoes of Lilith.
MacroBioBoi released a video where he’s talking more about this weird interaction/bug.
There are some other issues pertaining to the Barber. If your party member has one equipped, you won’t see damage numbers. All the damage will be displayed only on the Barber’s user screen. Your skill effects won’t proc and you will only see the “Immune” message then mobs will suddenly fall dead.
Barber Malignant Heart is killing co-op experience
by u/Most-Orchid-1079 in diablo4