Why this group is larger than the nesting elements?

[_Bluebug]

I did not rotate anything, there are no hidden elements. Yet, the resulting group is somehow larger than the nesting elements? Why is that?

test.afphoto

[NotMyFault]

Probably the nested gaussian blur (even with protect alpha), 

and add noise on a vector shape.

Add noise impacts the alpha channel, too. And vector shapes with nested filters on alpha never worked correct in Affinity: you get random bounding boxes.