The suspect would be the idler tensioner or the water pump itself since it too is driven by the timing belt. Maybe the bearing failed and is allowing wobble on the drive roller, causing the belt to walk. At this point the Water pump, tensioner, and cam shaft pulleys are are suspect until you determine which one or ones are bad.
By the way did Lexus do this work?
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At lexus Now. we changed the water pump, don't know if they did the pulleys' and tensioner. Thanks