Has anyone experienced a web lead(Form fill with all fields marked as mandatory) coming into Marketo without any data/just one field (first name)populated. what could be the reason for such leads.
Are you using Marketo landing page on website or only the form on third party web page. I have number of form which are used on third party website but I never faced any such scenario. If possible, can you clone the form/page and provide the link so we can check and help you.
If the person is browsing with JS off (unlikely, but possible) or forges a form post to get around your validation, they can pass any data they want (this is the case with all web forms, not just Marketo's).