This is quite confusing behavior: say we have 3 tasks:

• 1 in group A
• 1 in group A and B
• 1 in group A and C

Then we do a query to show all tasks in group A: we get the 3 tasks. Now we ask to group the results by group: we only get 2 tasks:

• group B
  • task in groub A and B
• group C
  • task in groub A and C

The task that is only in group A is gone from the results: it should be a in fake group called "Others" or something. Hopefully this is easy to fix.