You really should rename your field names with a hyphen (-) instead of an underscore. There is no solution for that other than replacing the underscores (_) for the hyphens (-)
I read somewhere that the problem is with the new php naming conventions and when used in functions, it separates the words when an underscore is used. And since WordPress uses php in its core functions, it is still a bug.
I am sorry, that your problem couldn’t be solved, but if the form is the only place where your fields
