Same thing just happened to me. Activated alpha Broodmother and was teleported into the ocean outside the cave (Fjordur). Luckily I was using Shadowmanes so they didn't die. I was able to attack and kill her but she didn't fight back and no minions appeared so technically it was the easiest fight ever but it's not good. A couple of sharks tried getting involved too but that wasn't a good idea.
Not sure if this is a Nitrado or Wild Card issue.