High Sierra quicktime player does not support H.265 videos?
“All iOS devices running iOS 11 and all Macs on High Sierra will support HEVC playback”
https://www.macrumors.com/guide/hevc-video-macos-high-sierra-ios-11

Use QuickTime to encode the video to HEVC/H.265 so you can ensure it is encoded to the correct standards. 
https://developer.apple.com/library/content/documentation/General/Reference/HLSAuthoringSpec/Requirements.html

I’m guessing you were encoding using Handbrake.