See QGraphicsItem::boundingRect it clearly says:
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Although the item's shape can be arbitrary, the bounding rect is always rectangular, and it is unaffected by the items' transformation.
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So group->boundingRect() should return always the same value independently of own transformation.
BUT if children transformation changes then group->boundingRect() can change (in case of one item MUST change) since area occupied by the child (in coordinates of group) is changed when transformation is applied on child.
It is like changing area of painting.