In my tests, it happened the first time you change business partner but the second time it does clear the location field in the Project window. It seems to be caused by the changes from IDEMPIERE-3961, particularly the if statement in the line 2795
The lookup is not refreshed with the change, so it contains the old value and sets it back to that.
Test case:
Window Project
Navigate to the project "Landscape"
Change the Business Partner to "Patio Fun, Inc."
The fields Partner Location and User/Contact must be cleared - they are keeping the old value
This is a blocker because it can lead to data corruption in many windows that have these kind of mechanism.
Regards,
Carlos Ruiz