Pretty sure this has been an issue for a while, but pets die during the Zhaj'hassa fight in vMoL and I believe it's due to them getting cursed and not being able to cleanse (I'm not sure how to really prove this, but it seems to be 30 seconds after others get cursed). Regardless of what is killing them, pets are supposed to be "invincible" in dungeons and trials and this hasn't been fixed for a few patches now since at least Summerset (I wasn't doing vMoL before Summerset, so I don't know if it was an issue then as well).