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This is a well known issue with this machine design. The nemas do not maintain torque when the power is off, so sometimes gravity will cause an axis to 'slouch'.
Mine used to do this, but for no apparent reason it now does not. The only difference between then and now is that the nema motor on the side that was dropping burned out and has been replaced. So there is likely a correlation.

As mjedlinski has pointed out: there is a modification available to remedy this, that involves adding another sensor and reflashing firmware.
In the interim you should just check that your X-carriage is level before you turn the machine on. Unless your printer is being moved/bumped/vibrated while the motors are unpowered (even if the machine is on the motors are usually unpowered when it is not actively printing) it should maintain level anyway.