Very odd thing happening here. We have several custom post types and several custom field groups on a site. One of these is a custom post type called "States" which has a custom field group called "State Data." This has all been working just fine for many months.
Today, I tried to add a new field to the "State Data field group. The field gets added, but then under Settings in the field group, the post type changes to "Posts" rather than "States" (screenshot: https://d.pr/i/ZOpG1w).
Dropping down the post type list there, I can see the States cpt, but when I select it and save, it reverts to posts.
Oddly, this only happens on the "State Data" field group. When I add a field to another field group that applies to another cpt, the field is added and the correct cpt stays in settings.
I have tested for plugin and theme conflicts and the issue persists even with all plugins inactive other than Meta Box and Meta Box AIO and the theme switched to Twenty Twenty Three.
I had the exact same problem. I increased max_input_vars and it solved it.
Surprising because I didn't think I had reach the limit of '1000 inputs' already (by previous value). I have less than 100 custom fields on that page alone, so I don't know what counts as 'input'. But anyway, it's solved for me.