Certain mp4 files not playing via Mac OSX Mojave, but play fine on iPhone?

I have been having MAJOR issues with certain .mp4 files since the Mojave update. Certain .mp4 files work on occasion, and certain files do not. However ALL files will work when played via iPhone and/or AirDropped to iPhone from said iMac.

The codecs are HEVC and AAC.

What is the deal here? I am assuming it is something to do with the .mp4 container? This never use to happen before the update.

When I attempt to open one of the files that does not have a preview generated, Quicktime tells me it "cannot complete the operation".

I am not looking to transcode these files via Handbrake or some other alternative like that. As stated - this use to not be like this.

I have contacted Xbox Support and they have said it is absolutely something on the Mac side (hence the iPhone working and the desktop not working).

Any help/insight would be greatly appreciated.