There is a confirmed bug / exploit that tanks get in PvP.
https://twitter.com/holinka/status/849381814871392256
They've been getting reports of it, but they say it's "complicated". I have ran into many tanks since 7.2 that take 0 damage. I believe some people have reported it and GMs asked them to stay out of BGs until it's been fixed (this was on the official forums I saw, but I don't have the link). Theres been a few hotfixes for it but obviously it's not 100% fixed. And there are certainly people knowingly going in while glitched and taunting the enemy team :\ A lot of the tanks that have this bug say a relog fixes it.