@joeyk will be able to explain this better than I.
When someone visits a site where the Mautic tracking code is installed, Mautic tracks them as a simple IP address via the 1st party cookie that is dropped by Mautic. When that anonymous contact completes a mautic form, all the history that was collected for the anonymous contact is attributed to the now know record, and future return visits are attributed to the person as well.
With the current 3rd party form integrations, while the contents of the form are passed to Mautic, the association between the tracked anonymous IP and the now known contact is not made. Meaning the historical browsing history is not allocated to the new record, nor are future visits. In fact Mautic will now have 2 records for the same person as Mautic will continue to track their visits as an anonymous record parallel to the contact that has been passed in via the integration.
Thsi post may explain this better. Passing a lead from One Mautic to another on Open Mailer - #9 by joeyk