Secondary belt popping off

Mower had been working fine this season, but when I was sharpening the blades, I noticed the secondary belt (one that goes around blade spindles) was worn, so I decided to replace it. Found a belt on Amazon that was touted as a replacement for the exact part number, and I checked the belt after receiving it to verify that it was proper length (width is slightly greater, but only because of the old one's age) - seems to be the exact same thing.

Here's my problem, though: When I try to start the blades with the new belt, the belt comes off the tensioner pulley. I have followed the instructions to the best of my knowledge- I have checked the routing against the book and a picture of the mower with the old belt on it. I have tried this three times, and this problem happens at low and high throttle. The belt seems tight when the deck is on the floor, so it does not appear to be loose or anything.

Tractor is x540 with a 54c mower on it.