I gave up on this.
I have the Apple Lightning/SD card reader (most recent version) and tried to upload Mavic video and still files to an iPhone 7Plus and an iPad Pro. The card is recognized fine in Photos on both. Usually the videos and still import with no problem, but sometimes I get an error saying that the video file is not supported after importing the files.
I THINK the problem is that the iPhone/iPad tries to upload the files to iCloud after importing, and that's where the problem occurs. I haven't done specific testing of this, but anecdotally when I import the files when I'm NOT on WiFi they are fine because the devices don't try to upload to iCloud unless they are connected to a WiFi network. When I import them when I'm on a WiFi network, most of them become unreadable on both iOS and MacOS devices.
One possible explanation that I have not fully explored yet is if I try to open the video files before they are fully uploaded to iCloud vs. not opening the files until they are actually fully uploaded. It's possible that viewing the video files while they are uploading causes problems.
EDIT: I have this behavior with both 4K and 2.7K videos, BTW.