Hi tcmanders,
I found a strange thing:
As we have seen, a form submission record is actually a Form Submission entity record.(logical name: msdyncrm_marketingformsubmission)
At first, the Submitted Value does exist in Form Submission entity, because I found it in customization.
I create a custom view for the entity, and add submitted value as one of column, it displays well.

However, when I export my custom view, the field is still blank as issue you encountered.

So I did retrieving with Web API to check whether the field actually contains data,
when I retrieve all records, the field contains data:

then the strange thing happened, when I retrieve a specific record(first record in above screenshot), the field became null.

So in my opinion, it could be a bug at present which caused such issue.
If you would like to do analytic for form submission data, as a workaround, you could enable 2020 wave 1 early access for your environment,
this release has brought a new feature: export insights data as csv file.
By this way, which means that we instead export data from Insights widget.

Submitted value can display normally in exported csv from Insights data.

Regards,
Clofly