Altium: unable to slice a polygon pour on a mechanical layer
Polygon pours are meant for copper layers so don't behave on mechanical layers.
I would suggest working backwards... draw your 'cut' on the mechanical layer using a line and then add a note to the manufacturer.  (Mechanical layers aren't copper so won't be treated by the PCB house as anything but notes).
An alternative is to draw the two (or more) stiffener regions using Place-&gt;Solid Region but draw then around the cut marks you've already placed.