Ok, I’m really shitty now, cause I’ve wasted your time. I did the old disable all plugins except WP Show Posts and Advanced Custom Fields and it seems that the problem was the Codeit Glossary plugin which somehow is preventing the custom field from appearing. Makes no sense but there you go! I’ll work at fixing this issue by troubleshooting the Glossary plugin with their dev. There plugin is great in theory but it has so many quirks.