GRASS GIS not quiet when called from Python although quiet option is set

Question

I am working with GRASS GIS 7.4 and I call it via Python and GRASS session.

If I run this command:

r.cost(input='costraster', output='costsurface', start_points='start', stop_points='stop',
       outdir='movementdirections', memory=ram, overwrite=True, quiet=True)

I get loads of console output although I have set quiet=True. Why is that so? Also it is in red color...