Why does iMovie on iOS 11 cause imported video to become unsupported.

On my iPhone 7+, using iOS 11 and newer. When I import video from my dash cam or other sources using a camera kit adapter they work at first in photos. After opening I movie these videos show a cloud icon (in iMovie) and then no longer work in photos. They now show a circle with a !, and say unsupported wedge. Trying to play.