I assume it plays fine in MC? What version of MC on what platform?
What it in the compression filed for a file that doesn't play?
Number of channels and sample rate?
I think there is confusion here:
JRemote (latest) does not play HEVC encoded video, regardless of container (mp4, mkv). Audio can be heard but video does not work. It shows the old "quicktime logo" when playing to the device (iPhone or iPad, latest iOS/iPadOS versions).
Mo 4 Media plays HEVC encoded video fine on iPhone and iPad.
I would suspect that whatever video engine you are using for JRemote on iOS/iPadOS is outdated/old and does not support HEVC.
As consequence, Airplay to an apple device, in this case AppleTV means no video there either, since it's decoded on the device and then streamed to the AppleTV.