Diablo Overly Targets A1 Mercs

I was playing through single player, and as I was fighting Nightmare Diablo, he overly targeted my A1 merc. He would cast bone prison over and over and over on her for the first 15 seconds of the fight, then he ran to her, ignoring me and my valk. and proceeded to continue chasing her or lightning bolting her. Once she died, he went back to ‘normal’ and focused my valk, as she was closest to him. My valk was the closest target to Diablo the entire fight, and he ignored her and chased my A1 Merc, seems extremely odd.