The code in KisBrushMaskScalarApplicator (which I recently patched so that it becomes a bit faster) is not aware of LoD, so it ends up sampling the individual pixels it'll display from a uniform distribution.
Instead, it ought to sample from a "bell curve" that gets more narrow as the zoom level moves away from 100%.
The problem I face in implementing this is that the MaskProcessingData currently holds only a KisFixedPaintDevice, which knows nothing of the KisDefaultBounds type. I cannot simply fake a KisDefaultBounds object in that code; it has to be passed in from the callers.