On further thought there could be a reason this is particularly happening with BlendMode.SCREEN
. It's the only blend mode that uses Context3DBlendFactor.ONE_MINUS_SOURCE_COLOR
. What that means is the amount of the destination that shows through is based on the colour of the thing being drawn, not alpha. The blacker it is, the more shows through.
And premultipliedAlpha
affects colour, or more precisely it determines whether the colour data in a semitransparent area includes a factor for how transparent it is. As it affects the colour it could impact how BlendMode.SCREEN
is drawn, in semi-transparent areas so near edges.