Then the app is not yet optimized for HS. Same thing with pro apps like FCPX, Compressor and Motion. Not yet optimized for HS . Try iMovie it is optimized for HS.
I think the issue is a hardware limitation. Only the 2016 and newer machines can decode HVEC video properly. but you should still be able to play it with software decoding only.
Then the app is not yet optimized for HS. Same thing with pro apps like FCPX, Compressor and Motion. Not yet optimized for HS . Try iMovie it is optimized for HS.