subitems not grouped in right group or parent task

HI, I want to group my subtasks under the parent tasks under each of the groups of my project.

However, looking through previous discussion, I found the following settings will do this:

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However, when I do this, all my subtasks from all the groups end up under one group which has the label of another group. See below (the second picture doesnt give the list justice - but there are over 100 subtasks in there belonging to other groups):

image

Is this a bug, or is there another setting I need to apply?