When the geometry of the sculpt mesh was replaced when restoring from
a full undo step, the runtime data was not cleared (including any
normals, triangulation data, or any other cached derived data).
In the report, only the invalid normals were observed.

The fix is to simply clear these caches. Later they will be reallocated
and recalculated if necessary. Since the whole mesh replaced here
anyway, this should be a safe fix.