InfoPath Dev

I have a library with a Person field, and a corresponding form field. How do I get the form value Promoted to the library record when the form is updated/submitted? I can choose any of the 3 elements of a Person as the source of a Promoted property, and am currently choosing the AccountId, but the value does not appear in the Library record. (I know the value is there b/c it is promoted into a different field, which is of type string.)

Unfortunately, this is not supported for promoted properties. If you are using SharePoint Online, you could use Flow to set your person group - promote the account id and then with a Flow on the library, populate the person column using that information. The only downside of this approach is that you will be dealing with the polling lag in Flow - it may take a little time for the Flow to pick up the change and run (up to 5 mins last time I checked).

I should add - you could also use the SubmitToSharePointList functionality in FormsViewer to update this data after submitting. It is a little tricky because the mapping tool doesn't currently support libraries, but I can give you a hand with that if you need it. Then you don't have to worry about the Flow polling lag! :)