A friend did his major 100K service (Canadian mileage), and as part of this he did had a new serpentine belt, idler and tensioner pulley, and all sourced from a local parts jobber place to save some dollars (I totally get this, too.....however!!!!!!!!!!).
Nothing fit right at all, and misalignment was abound.
I suggested that he go with all new OEM factory parts from the dealer. He did, and the world was right afterwards!
When I did my major 100KM service, I went with all new OEM dealer parts and never had an issue whatsoever with my belt nor my idler/tensioner pulleys.