Quicktime and mkv/HEVC

QuickTime Player has never listed MKV support, including the latest version.

On opening an MKV file in QuickTime Player, it will try to convert it to MOV, initially as a temporary unsaved file. This conversion may or may not succeed, depending on the properties of the source file.

You may be better of in re-packing the MKV as MP4, which can often be done much quicker with passthrough of the video and audio streams.

Hello, Hajado!

Thanks for reaching out to the Apple Support Communities! That’s a great question you have about using QuickTime to natively view MKV and HEVC media. Check out this article Using HEIF or HEVC media on Apple devices. Specifically: 

"If you open an HEVC video in QuickTime Player on your Mac, you can choose File > Export As, then save to H.264 by making sure the HEVC checkbox isn’t selected."

Hope this information helps!