Can you please clarify your question? Do you want to validate the field value on the server side? You can try to use the remote validation as in the documentation https://docs.metabox.io/validation/
Yes, I'd like to validate the field on the server side, but while submitting/editing the form in the WP admin, not from the frontend (on the frontend I can use the rwmb_frontend_validate, which doesn't work while in the admin).
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