iPhone X - Your device may differ!
There is a work around for the issue of not being able to select photos/videos that sync to the Lr CC ecosystem.
1 - Disable Auto Add Photos and/or Auto Add Videos in Lr Mobile General Preferences
2 - Shot with the Mobile native camera which puts images into Camera Roll.
3 - Use Camera Roll to delete any rejects.
4 - Use Lr Mobile app | Click the menu (. . .) | Add Photos | Camera Roll | Select images | Add xx Photos (bottom of view)
Works the same with videos.
Since the Lr camera doesn't shot video, using the native camera is my choice for use - photos and/or videos.