Ahhh! Unfortunately that’s common in acrylic when you have the design come to a sharp point. (Like the crotch there.) The acrylic is under stress, and when you hit it with too much heat, (the beam pauses for a split second longer when it has to come to a complete stop and back up), it can develop splits or cracks at the point.
You can apply a design device called “dog bones” into your corners…basically it’s a tiny circle that redistributes the forces and keeps it from splitting. Or you can try rounding that corner…it might be enough to keep it from splitting.