When cutting triangles from a square if you cut once from corner to corner the long side is a bias. If you cut twice from corner to corner the 2 inside cuts are bias and the outside is a straight grain. When I teach I tell my students to put a pin or some kind of mark on the straight of grain edge so they know which is the straight of grain when putting pieces together.
I know this won't help current problem but it may save frustration in the future.
Longarm