Theres only one quartz i know of thats guaranteed for ufh and thats the stuff from Esha Stone (94%, 12mm). But its limited to 150w only and must be encased in 10mm slc before tiling and limited to 27C on the stat. But even then i have my doubts.
Don't know what quartz that is but looks like the cable would have been touching the base of the tile. Any quick repair job is highly likely to go the same way unless its all ripped up and started again. But would be a safer option to go with a mirror flecked granite.