Hi David,
I’m having the same issue (CLS caused by featured image on post page). However my Customizer > Layout > Blog –> Featured Images width and height fields are empty.
The Media Attachment Size is set to “full”.
I tried disabling lazyloading for the image with code you posted in another thread, but it still persists.
When I throttle my connection, I can see the post loading and only after the featured image loads and moves all content below it lower.