Unable to change default build definition where one definition is a PROJECT definition and one is a GROUP

This is perhaps related to CONTINUUM-1410:

We create a MVN2 project in the usual manner within a predefined project group.
We then added a non-default build definition to the project - the GROUP definition continues to be the default.
Sometime later, we wanted to temporarily change the default to the PROJECT definition.
When attempting to change the default back to the GROUP, the default field for both definitions (from the Project page) is read-only set to true.