Group By and Filter don’t work properly together when you need to filter a task by duration.
The group by method accumulates tasks on the project type if they’re filtered out via onBeforeTaskDisplay (the duration of the project type include a sum of the total duration of all children, even if all children are filtered out and no tasks display under the project).
The duration of the project type needs to handle filtered out tasks and not update it’s duration sum accordingly. This seems like a bug.