Repro Steps:

1) Create a Page
2) Introduce a Rectangle to the RootGrid of the Page, of Properties:
Width=128, Height=256
Margin=0, 0, -512, 0 (-512 on Top Margin)
HorizontalAlignment=Center, Vertical Alignment=Center

3) Introduce a second Rectangle to the RootGrid of the Page, of Properties:
Width=128, Height=256,
Margin=0, 0, 512, 0 (512 on TopMargin)
HorizontalAlignment=Center, Vertical Alignment=Center

At the end of this process, you should have two Portrait orientation Rectangles, each at equal distance from the center of the page.

4) Multi-Select both Rectangles, and right-click either of them to access its context menu. From within the context menu, navigate to 'Path', then select 'Unite'.

5) Select the new compound Path, and navigate to its Properties Panel, and open its Render Transform 'Origin' tab.

Observe the Origin value of "-0.004, -0.001"

Expected Result: The Origin of a newly-formed Path as a result of Booleaning, should be center-center (ie "0.5, 0.5").
However, there is a different ticket for Origins not defaulting to Center-Center across all object, so until that is fixed, it should likely take on the current default Origin value for all elements of "0, 0"