It is expected behavior that the Opportunity subtab on a Project Record disappears when Advanced Projects is enabled.
To show the Opportunities linked to a Project record, the solution is to create a Custom Sublist for Opportunities that is linked to Advanced Projects.
1. Navigate to Lists > Search > Saved Searches > New.
2. Select Transaction
3. Fill out the Search Title field.
4. Public = T.
5. Available as Sublist View = T.
6. Criteria tab:
- Type = Opportunity
- Main Line = True
7. Available Filters tab:
- Project(Main Line) : Internal ID
8. Save and apply this as another sublist.
9. Navigate to Customization > Forms > Sublists.
10. On the Custom Sublists page, select Entity tab.
11. On the Search Column = Add the Saved Opportunity Search that was created.
12. Label Column = Put a name for the Sublist.
13. Apply the Sublist to the Job/Project column. This can also be added to a Tab.