Maybe it has something to do with thrust forces. That high helix is trying to suck the spindle out of the head-there's no upward thrust on the spindle bearings (preload lost?). I think you'll have to take an extra pass on the bottom or change tools. You might try putting a major hone on the cutting corners. With a hand stone grind the corner back so you have .010-.020" netural (0 deg.) rake on the very bottom of the end mill. Same as you would do to a drill that grabs when you break through on a piece of brass or stainless.
Jim
www.picopascal.com