Slab blocks' orientation is not accounted for in symmetry mode unless 'mirror cubes' is enabled:
{F4325}
As a sidenote: there are more instances of using symmetry mode where you'd want mirrored cubes on as opposed to off when working in symmetry mode. Perhaps the most expedient solution, if slabs are to be treated as faced blocks and not as shapes like wedges, etc (all of which auto-symmetry regardless of mirrored cubes' toggle position), is to have mirrored cubes turned on by default alongside odd-sym mode.
i would also like to couple this report with a subtle reminder that corner shapes still do not mask out with the replace filter with their orientation intact, as a related issue, making repaints a chore if you mis-click on builds that use them in quantity.