I'm maintaining a friend's website, and after updgrading to WP 5.5 some custom metaboxes on custom post type edit pages stared disappearing on save, but not all.

There's a custom post type - 'Releases'. It has four custom metaboxes added by the theme. When you click on the 'Add new' button, you get a regular editor page with all four metaboxes visible. They're also visible on 'Release' posts last saved before the update to WP 5.5. But as soon as you save a draft or publish or update any of the 'Release' posts, two of the four custom metaboxes disappear. The entered data, however, is saved to database. The bad thing is once you save, even just that one time, you can't edit the data in those metaboxes.

I've installed the jQuery Migrate Helper plugin, but that didn't help. Enabling WP_DEBUG and WP_DEBUG_LOG didn't show anything wrong. I'm at a loss. Theme hasn't been updated in a while, but WP and all plugins are regularly up to date. Disabling plugins one by one didn't help either.

Any pointers in the right direction to solving this greatly appreciated.

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