This happens only for mesh refined runs and on reflevel>0 ie in the buffer zones. This happens only if the "useghosts" option is not set (it is unset by default). This behaviour is technically allowed by the docs since Slab only claims to fill in ghost zones and the outermost points are not ghosts but buffer zones (though Carpet disagrees somewhat on that in that it does not count them in buffer_width).
Anyhow, this does not to my knowledge actually affect RotatingSymmetryXX which are the most prominent users, at least their poison_boundaries option does not seem to pick this up.
To test, run the attached modified slabtest.par test file whcih fails in the identity test for level 1 unless useghosts=1 is used in slabtest.c (and fails int the skip-every-second point test if it is used).