A simple transform node in sbsar filter messes the height?

Why  a filter with just transform node  in sbsar   turns L16 Height channel into looking like 8 bit  one with stepped  gradients?   What am I missing?

if set the filter off it's back to normal .

The filter is basically  input>2dtransform>output.

  Same sbsar works just fine  in Designer or Sampler with perfectly smooth height gradients.   Same happened when I tried to use fx-map to control transforms instead of transform node.