This has been happening frequently enough that I am here now. This is when Brann is tanking. He will maintain aggro, but his DPS completely drops to less than 50k when it was averaging about 800k. You look at him and he is completely motionless. Completely random, but it happens frequently.
Also, with my restos, he keeps running away when I drop my circles of healing. He will take off and run out of range, taking the adds or bosses with him. I can’t keep a circle on him to heal him.
Brann’s tank spec did get adjusted during March/April, so it is likely why you are noticing DPS drops. Unfortunately this is beyond what Technical support helps with, but there was a blue post. You could edit this post to move it to bugs, but not sure if this is a bug, and simply was because the dev team felt the tank spec was too powerful at DPS then intended.