I moved all of the vertices the that are beyond the line I wanted to cut, onto the angles of the square...
That way it's as accurate as it gets and it even saved me some unwanted polygons while extruding the edges of the square...

Anyway, does someone know why this bug is happening?
In some cases the split tool works fine while cutting through several faces...
I think it has something to do with the plane of the edges (It shouldn't be even) , because when I tried to cut a sphere or a curved panel (Polygon Modeling Tutorial - Helmet's face shield) it worked just fine.

Also, can I expect a patch or something of that sort?