A warning. Because you built the laser, you know what I’m about to describe cannot possibly be the problem.
Most likely, there’s a loose setscrew somewhere between the X axis motor shaft and the laser head. It’s supposed to be snug on a shaft flat, but when it’s a little bit loose it can stick at any position between two extremes depending on the history of forces applied up to that moment.
As long as it’s stuck, the laser will seem to work perfectly without losing its position. It will become unstuck only when the forces from motions exceed the stiction holding the shaft in place. Those forces are highest during vector motions, rather than engraving, so the vectors are most likely the part of the layout triggering the problem.
A previous discussion has pointers to track it down:
Good hunting …