MMike, I believe it to be the power-of-two strikes again. But nothing but my own testing, and educated guessing to back me up.
If you are using the draw_line3d process to 'visualise' the detected edges of the sprites collision box, try rotating or tilting the sprite by 45 degrees or more...
Could this be called a bug?
"There is no fate but what WE make." - CEO Cyberdyne Systems Corp. A8.30.5 Commercial