Help! Anonymous activity not backfilling once person is known

We recently noticed none of the people filling out our demo request form have any website tracking that pre-dates the form fill. Our understanding from Marketo product docs is that anonymous web visits are tracked in the background and then should populate in the person record (and Sales Insight) once the person fills out a form and becomes known. This is happening as expected on our content form. Our issue seems to be specific to the demo form. 

Has any one encountered this? Any suggestions on how to fix?