The Axial Symmetry condition only applies to straight boundaries that are both on the axis of symmetry and adjacent to a domain in the selection of the physics interface. If rays are propagating in the void region outside of the selected domains, it is possible for the rays to enter the region where r < 0, even though no geometry is defined there.
To ensure that rays cannot propagate into the region where r < 0, draw line segments on the parts of the symmetry axis where no geometric entities exist, then add the Wall node to these edges. Keep in mind that rays can only be subjected to boundary conditions if the corresponding boundaries are meshed.
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