Pre-compiled form field value in mandatory field counted as if user filled

Hello.

I am experiencing an unexpected behaviour in forms. I set a pre-compiled custom field, to indicate to users how the field must be compiled (serial number of 9 digits).

Despite I set it as mandatory, if users don't fill it, when submitting the form it is accepted. Afterwards, the field associated to the user has the value I set as pre-compiled example.

What should I do to avoid this problem?