Two Three bugs with paint mixer brush.
1. Go to paint mixer brush
2. Select "transparent" with the small button in Swatches Studio.
a) go back to normal brush, you will; see white colour in swatches but it's actually transparent.
b) add fill layer, it will be filled with transparent but the bar on top will show some colour (white in swatches, grayish if you open the color selector in the top bar)
Create a fill layer, if you use the colour mixer brush you can see it's generating history events (and using CPU cycles heavily) but there is no effect.
Either show a info about this or rasterize via assistant.
I also got one more, but I forgot It. I'll add later.