Hmm, interesting. I was able to replicate that as well using an MC6x15.3. I think the reason for this is because the flange on the MC6x15.3 doesn't really end with a vertical at the taper - the radiused end of the flange ends with a curve. Something like a C6 does have this. Here are the ends of each, and if you zoom in while the display type is "Exact" you will see the same thing.
My guess is that because Advance Steel doesn't see a "flat" side to that profile on that end, it searches for the next nearest edge.
The next best option is to cut the corner using a plate and Element Contour along the UCS of the cut as shown:
