Slab blocks' orientation is not accounted for in symmetry mode unless 'mirror cubes' is enabled:

Quality-of-life feedback from a longtime player:
There are far 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), would be to set mirrored cubes toggled "on" by default, alongside odd-sym mode. I rejoiced when that toggle was set to default for every sit-down and I did not have to check it anymore.

I would also like to couple this report with a gentle prod that corner shapes still do not mask out with the replace filter with their orientation intact, as a related issue, making ship repaints (something I foresee players doing a great deal of in coming months as new hull colors are added) a real chore if you mis-click a replace on builds that use them in quantity (a use of the undo function also undoes corner blocks' original orientation).