Hello Guys, a common issue that i have yet to find any solution, even if i changed two buttons instead of one button, the corners will still kind of flip upward unless the buttons are really close to the corners. has anyone come by a solution so far?

Yes. When attaching the lining to the flap(and only glueing along the edges, not fully down) You need to stretch or pull the corners of the lining out just a little and then press it down the the exterior. this creates a slight tension that will make the ends of the flap slightly bend in creating a nice shape.