When a record is converted in SFDC from a lead to a contact Marketo automatically changes the SFDC Type field from Lead to Contact.
The change in SFDC type doesn’t cause a data value change so the Data Value Changes trigger and filter won’t work for SFDC Type. You will need to use the Lead is Converted trigger or Lead was Converted.
Alternatively you could use a different field such as Lead Status = Converted.
Make sure your lead custom fields are mapped.
During the conversion the Marketo ID stays with the record converted to a contact.