When adding a video from a photo to a playlist, it is found that it cannot be added to an existing playlist because the existing playlist is not displayed.
Are those both playlists in first image and you are trying to add to second one?
the entirety of photo libraries is probably going to be revamped soonish. I don’t think this is likely to get fixed until that is done. I unfortunately do not have an ETA
The first image illustrates an existing playlist, and the second image illustrates the inability to display an existing playlist when adding a video from a photo to an existing playlist.