Briefly, to reproduce.

• Create two groups, Group A and Group B, add some users to each (one each will do)
• Create to groupings, Grouping 1 and Grouping 2
• Add Group A to Grouping 1 and add Group B to Grouping 2
• Create two activities, say two Online assignments called Assignment 1 and Assignment 2. In each case set them to separate groups.
• Set Assignment 1 to using Grouping 1 and tick 'Available for group members only'
• Set Assignment 2 to using Grouping 2 and tick 'Available for group members only'
• Set up course completion for the Course requiring that both of the assignments be completed (there isn't any other option that forces all available activities to be completed)
• Add the completion block

Having done all that log in as one of the users. They can only see the assignment in 'their' group. Complete the assignment. The completion block now shows that the activity is completed. BUT... the block shows that the other actvity (which they cannot see and don't know exists) is incomplete. Pressing the 'more details' link even shows the name of the hidden activity.

Really, the block should not show activities hidden to the user. In addition these activities should not be part of the aggregation for the purposes of course completion. When Groupings are used in this way it becomes impossible to complete the course.