Didn’t work for me. Seems to be different every time I go into it, and this is with the new code snippet.
If I create a brand new custom post with no child posts I can add a featured image. However if I save and go back, remove the featured image I can’t add a new one.
Also if I add a new child post I can’t add an image, but if I save, come out and back in, I can.
It only happens on posts with children.
I’ve made a screen cast here:
https://screencast.com/t/wukWkSLj2hzf