I can confirm that HEVC support in iOS/macOS is currently a disaster.
Your iPhone X will convert the HEVC video to H.264 when airdropped, even to a modern iPad or Mac running High Sierra. Also when using Image Transfer on the Mac. The only way to keep the videos HEVC are to use the Photos App, according to a third tier Apple Support specialist. (Who also told me this was intended behaviour, because of course Apple doesn’t have bugs in their software
I had an iPhone X for three weeks in November/December and decided to return it becuase of this (and other bugs), so it’s possible that is fixed by now. I’ll try again when the new phones come out in the fall.