The more and more I play, the more and more I think you might be right. Or at least on the right track. Maybe not coded to cast against 2nd on aggro list, but probably coded to cast against the hunter. It definitely feels intended at this point, especially with all the radio silence about it on their end. I really think it has something to do with delves, since delves are a way to get great vault rewards and progress. I think they wanted to make it so hunters had to rely on Brann for healing and not damage, so that we arent OP in delves. It really sucks in the open world though.
I’m sorry 
it does really suck. I hope we at least get an answer or something soon. It’s really frustrating.