Just have had an issue with print quality where Y axis layer's rear end surface was shifted sometimes by 1/4 mm, front surface fine. Supposed loose belt, tightened it and result was much worse. When removed belt to inspect it if something was caught and started blocking it I've realized bed carriage is running like old rusty bearing. With belt on I've though it is just stepper. Added drop of silicone sewing machine oil per balls row (4 per bearing) and realigned them. It was preassembled in kit so never touched it but apparently should.
By the way Y belt fitting to carriage is way too high, will have to print new fitting.
So if you have print quality issues check bearings.