A theme I'm using came with several Meta Box integrations/developer modifications (things like setting a subtitle, changing post layouts, etc). I've since bought a developer license to add some more things of my own and migrate away from ACF.
All the developer meta boxes show correctly on posts, but whenever I create one it doesn't show in the post editor screen - if I go to the Screen Options setting at the top of the page and toggle on/off whichever metabox I created, it just shows a blank space of where it should be, but no box/custom fields.
I tried with the Builder extension installed, and with the builder function enabled via the AIO plugin, but neither worked.
It's something to do with when showing the meta box is reliant on certain conditions (eg in a certain category etc.) but appears to apply to any condition set. The meta box shows if I select to show it in all posts.