Quicktime Player can't open "moviename.mov".

Files downloaded from the cloud (amazon, dropbox, wetransfer, etc) will not play correctly in Quicktime. There is a black screen, but the audio plays correctly. A convoluted process of converting from a third party app will convert the file as a workaround but it shouldn't be necessary. These MOV files were created on an iPhone 11, so they are not unusual files. This is on iMac with El Capitan.

What will fix the problem?