Alpha masks do not render at all. The mouse however seems to interact with it as expected.
Numbered Steps to Reproduce Problem:
Apply an alpha mask filter to something.
Code Snippet (if applicable) to Reproduce Problem:
obj
icon = 'anyicon.dmi'
filters = list(filter(type = "alpha", icon='anymask.dmi'))
Expected Results:
The mask will render.
Actual Results:
It doesn't. (Your mouse disagrees though and will treat the areas that should have been transparent as if they actually are)
Rolling back to 513 solves it.