Since we can't put links/pictures in our posts here now...
I created a video to show you what happens.
- If you add an attachment
- Then select it as a cover pic
- Then hit save
- If you then deselect it as a cover pic
- Hit save
- It reappears as an attachment
See attached Video
You're right, I just had this problem by posting in the forum, I uploaded two images, and published. I saw only one image attached, and as I had read your post just before, I understood immediately.