Update on loaded association not detected upon flush()

If an association is joined during fetching and then updated in-memory, the update is not detected during flushing. I made the following test cases:

Association is joined:

• change NULL → entity: does not work
• change entity → NULL: works
• change entity1 → entity2: does not work

Assocation is not joined:

• change NULL → entity: works
• change entity → NULL: works
• change entity1 → entity2: works