Definitely bugging out. I just did this boss and died four times because the vine-tongues weren't working properly. They'd grab you, stun you in midair a few seconds, then drop you back on the ground to die from the boss's stupid ground-covering AOE death mechanic. Literally the only vine-tongue that was working was the one on my right, as you enter the arena. And the only way to get it to pick me up was to run to a high point on the sloping gray rock directly behind it. So my entire battle was spent by that rock--get pulled to plant for huge death-AOE, jump down to ground and run up rock during the attack on the plant, fight boss while standing on gray rock, get pulled back to plant when boss goes to activate huge death-AOE again, repeat and repeat and repeat till he finally died. Couldn't use any of the fun plant traps on the ground because I couldn't get anywhere near them due to the huge death-AOE going off and not being able to get the vine-tongues in those parts to pull me up.
Log into your elderscrollsonline.com website account and file a bug report there since they ignore things reported with /bug in-game. They just fixed another Murkmire quest where progress was blocked by faulty collision (I reported it less than a week ago, but it'd been an issue for at least a month before that. I don't know if they just got lots of reports and finally fixed it, or if my report, possibly combined with other reports from the recent forum thread about it, got their attention at last). Hope it's fixed soon!