The issue is clicking between "Mixbox" & "Normal" in the app with unmixed paint shows two different starting colours which must make any comparison of mixed colours effectively void.
This is actually an important point of pigment mixing. The saturation gains and hue shifts when pigments get spread or mixed with white are beautiful. RGB linear mixing (the 'normal' mode) doesn't capture them.
And a clarification, I wouldn't call this RGB linear mixing, to me that's mixing in a physical light intensity space. Rather, it's alpha compositing in an sRGB color space, which is probably the most common mode. It's possible to verify this by setting transparency to exactly 50%, painting 50% black over white, and 50% white over solid black, verifying that the color is the same, and measuring that - it's very close to sRGB (128, 128, 128). Linear mixing would give approximately (188, 188, 188). Incidentally, this also shows that Mixbox doesn't retain neutral hue when mixing black and white - it gives something closer to (129, 127, 130), a slight reddish-purplish tone compared with neutral gray.