I reached out to support directly and they just told me to submit something here.
Basically, I've played this game for ages, and "done" the Volcanic Fractal achievement "The Floor is Lava. Don't Step In It" multiple times, just as the achievement description states ("You've completed the Volcanic fractal inside the Fractals of the Mists without falling in the lava."). Unfortunately, I've never gotten credit for it.
As somebody who runs fractals daily, out of the last ~15 times I've done that fractal, I've fallen in quite literally, one time... There are only a couple of spots to even fall into lava, and that's the end boss (you're on an island in the middle of lava, so rolling into it, or stepping on the small pools on the island), and jumping en route to the boss, both of which take effort to mess up once you know about the path to avoid it.
I've even go so far as to sit on the side of the island and not move, making no mistakes, and still not received credit.
I believe either it's bugged/glitched on your end, or the achievement text needs to be updated to say something like, "Whole Party must not fall in" or "Can't be caught on fire at the final boss" so I would at least know what I might be doing wrong, but as it stands, I've completed this fractal without falling in on several occasions for no credit.
I can't find anything online about it, which leads me to believe the mastery point should be very obtainable, but it isn't for me. What can I do to fix this? Thanks.