I've never experienced this. I killed Ifrit 32 times (with a lot more fails), killed Garuda 5-6 times (never failed this one, for some reason) and beat Titan 4 times with MUCH more fails. The only trolling I ever experienced usually came from DPS who were leaving. They would run towards the boss, pull it to the rest of the group and leave so it attacks everyone else.

I don't imagine this never happens as they can keep doing it repeatedly because someone else is going to leave before them, but as it has never happened to me or anyone in my FC and this is the first time I've heard of it, I doubt it's that common. Not to mention kicking tanks from groups probably won't go well as finding another tank can be difficult in regular dungeons and unless they've made it so you can bring new players to trials it'll be impossible there.