Same issue here, though possibly not as frequent. I also have the HP Spectre x360 15 ch004 with Vega M GL.
I absolutely love this laptop and how powerful the Vega chip is, however I am worried about a lack of support. I have tried various drivers, from Intel's NUC, to Dell's version, to the Vega drivers right from AMD's site. I also did a factory reset and loaded the stock drivers from HP again.
VIDEO_TDR_FAILURE issue persists through all of that.
I mostly play Witcher 3, though it has also happened with World of Warcraft and even the old Deus Ex: Revision
I suspect it has to do with the dynamic wattage component of the i7-8705G. CPU and GPU might fight each other for the juice.
Could you please furnish the link to the AMD graphics driver you found on the AMD site for Mobility Vega? Love to see it.