Good morning Johannes and welcome back!
Thank you for reaching out to us.
We are sorry to hear about the featured image code change not kicking in.
I would love to help you with this topic.
The good news is that it sounds like you are on the right track to achieve your goal.
We just need to find out what’s missing.
Could you please send me a copy of the latest version of your WordPress theme and child theme via a link to a .zip file link (stored Dropbox or Google Drive) so that I can run some tests on my end ? I recommend a private reply for that purpose.
Please ensure you are using the latest files as found on your actual website.
This way I will get access to any updates or customizations you might have made.
Hang in there!
Geoff B.