Every time a form is submitted, only the values present on the form at that time (either in hidden or visible fields) appear in the Filled Out Form activity. Previously filled fields that have been suppressed by ProgPro won't appear in the log.
That being said, there's no way within the UI to report on that data, as Marketo doesn't really treat individual Filled Out Form activities as first-class from a reporting standpoint. What I would do is pull the activities out using the Bulk Extract API, then you can do all manner of slicing and dicing and time series analysis.
Within the UI, you'd be limited to trying to see which fields changed (Data Value Changed) , but that's quite prone to error: a field can be in a form post but not actually change in the db, if you have other campaigns/API calls/syncs that change it in the background.
Also, to do this analysis properly you need to know when a form was submitted from a new session (where ProgPro doesn't work, by definition) vs. an existing session. And of course that's affected by which browser (i.e. browser vendor) people use, as Safari ages cookies out dramatically sooner than FF & Chrome!
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