Do you have “Post Type Switcher” plugin activated?
This is the same issue you are seeing: https://wordpress.org/support/topic/featured-image-is-blank/ and it seems that plugin was causing it.
I’m pretty sure it’s not the theme causing this, it is most likely a plugin conflicting with core WordPress functionality or another plugin.
Could you de-activate your acitve plugins one at a time and see if the problem goes away?