Hello,
Welcome to Microsoft Q&A!
After talking with the team, I have to say that this behavior is expected. The InkStroke of the default pen will always be rendered in full opacity. The InkDrawingAttributes.Color Property accept ARGB values but the value of the transparency component (A, or alpha channel) is ignored.
So you can't change the opacity of the InkDrawingAttributes for the default pen brush.
Thank you.
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