Featured image missing

Hello

Sometimes, featured images are missing in the child post, even if Custom Field is checked. But the featured images are well registered in the child Media library.
Even with SuperAdmin rights, when I publish once again the post with the featured image to update the child post, it doesn’t work.
I can send you the debug files of a child post with featured image, and one without.
It seems the thumbnail ID is missing, there is another line instead : _edit_lock’=> (and a number).
Thank you for your help !

Alexandra