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?