vtkMultiVolume::GetBounds() return wrong bound when sub volume set position

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I have one vtkMultiVolume which have three volume.
the three volume have same size and same pos. e.g. 200x300x400.
when not set position, GetBounds() will return right.
when the three volume set small pos(e.g. pos(10,0,0)), GetBounds() will return right too.
But when I set the pos larger than half, e.g. set pos(150,0,0), GetBounds() will return (50~200,0~300,0~400).
it should return (150~200,0~300,0~400).
I investigate it and find the function vtkMultiVolume::ComputeAABounds calc wrong bound.
Is it a bug?