P: Mixer brush preset: saved but if I adjust the size of the brush up or down the texture disappears

The Mixer brush scaling seems to be a bit buggy with saved brushes.  Reproducible in V19 and V20.  

With the Mixer brush, if I sample a texture and then paint, the texture scales up and down with the brush size when adjusted (man, would rotation be sweet too).  However, if I save that brush (either as a Tool Preset or a Brush Preset) when I load the brush again the texture is saved, but if I adjust the size of the brush up or down the texture disappears. It will revert to the last sampled texture/colour.  This only happens with saved Mixer Brushes, but it does make them a lot less useful.  Bug?