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Try not to push the Pulley on motor so deep. Also Sobo84's pulley may help as it have bigger hem on the top, that will keep the belt on it.
If the bearing on the other side is too high, you can grind off the plastic part a bit, so it will sit a bit lower.
Also check if the belt is not pushed too deep in the Y Frog. Try to make its side even with the bottom side of the carriage.