Block Editor – Meta values not saved, meta changes to empty array on update

So, in my case, the answer was simple, though it took a long time to find. Just in case anyone else ever has this issue, here is what solved it for me:

I was calling the register_post_meta in a file that was only being included if is_admin() returns true (this is a plugin metabox conversion, so there are some admin-only assets being called, etc)

Long story short: meta fields were being registered in the block editor because is_admin() returns true, but when the request was sent to REST, is_admin() is false, so the meta fields were not being registered, and an empty array was returned.

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