I was going to say it must be a bug but I tried and had the exact same issue. Found the answer here 4.4.5 and 5.0 Scratchpad and canvas have different effect of color smearing.
Try drawing on the initial white background layer, you’ll see you can get the same results. I will read on it a bit more to understand the reasoning behind it but I think it has to do with “Blending on Alpha” and painting over white fill layer interacts with the white of the layer, this should solve your issues.
Edit: Please let me know if this fixes it for you, I am quite certain it will but just as a confirmation for future readers as well