The Require Single Document per Document checkbox is not checked in AP Preferences. I also cannot find any customization or business event that would populate the Project field on the header of an AP Bill.
We have added the Project field (APInvoice.ProjectID) to the Bills and Adjustments primary list and here is an example of what we see:

These four records happen to be for the same vendor and there are additional records with company WESTMIN that show Project X.
The Detail line for both of the LPC bills has an active Project.
We are on version 23.210.0017.
If anyone has an idea why or how the Project in the header is being updated, please respond.