[plt-scheme] interaction bug of draw-bitmap set-scale and get-loaded-mask

Attempting to draw  a masked bitmap (mask argument of draw-bitmap supplied with get-loaded-mask) in a dc object whose scale is <1, leads to weird results.
But using a second bitmap as the mask, loaded with the same get-loaded-mask, works fine... ?

Since this is my first post here, I really want to thank all the contributors to PLT Scheme ! 
I've been using it compulsively for the last 4 years, learning scheme autonomously in my spare time...
There is something almost utopic about this project, that makes it extremely moving and precious...thank you all.