Recently, after moving numerous photos/videos from a folder like Camera or Screenshots to another folder and then later deleting those items (not just 'delete from device' but actually trashing it), the deleted media eventually reappeared as Cloud-only in Photos. So it seems that moving media between folders can cause the media to only be removed from the device when deleted, but keep the copy on the Cloud, when it should also be deleted from the Cloud.
Reported this issue to Google. If anyone knows the cause or a workaround, please share. Thanks.
Reported this issue to Google. If anyone knows the cause or a workaround, please share. Thanks.