I've always had the theory when tightening the lead screw set screws and the bed carriage isn't parallel to X it'll cause side pull force which will cause the bed to become misaligned when the power is off.

Loosen the bed carriage mounts and the lead screw set screws. I used 2 90° corner brackets. Press the frame so the brackets touch the X carriage. Tighten the M5s for the bed frame brackets

For this part I clamped the bed frame and corner brackets to the X carriage. Spin the lead screws into the coupler so they just kiss the bottom of the coupler. Than tighten the set screws on each coupler equally. If your couplers have 2 set screws that hold the lead screw don't tighten one completely, than the other. Tighten both of them equally.