I included a picture to explain the problem.
If the user intentionally chooses to draw inside a shape, then that limit should be respected. but instead, if we are painting with a color that exists outside, new strokes merges with the older ones, resulting on the painting escaping the "draw inside" feature.

I think that the blob Brush merging should be less aggressive and should respect masks, otherwise it's not intuitive and goes against user input, which qualifies this to be a bug.