I think you’re right that it’s related to the other issue (bug 521993).
Look at what the neutral point is set to by default: auto. Now try taking one of those black-on-transparent brushtips, unchecking auto neutral point, making sure it’s set to at least 127, and switching back to alpha mask. It works, while with 0 it’s semitransparent?
It seems to me the cause is that KisColorfulBrush::brushTipImage() doesn’t check whether the neutral point is enabled or not, combined with auto neutral point being 0 for those particular brushtips.