Summary:

Create a new Entity, for example Project, using the Advanced Customization Wizard.

In the context of a Project record, click New Case from the right hand-side tab. On the Case entry screen the Project field is not auto-populated.

The Project field should be auto populated. Once the case is saved, it can be found within the Cases list for the Project.

Workaround:
Within context of a Project click the Case tab. The Project field is auto populated on the entry screen.

Resolution:
This issue has been resolved in Sage CRM version 7.2b.