MEP802A Fan Belt Failure

I am having issues keeping the Micro V Belt on my Generator.
Unit had a water pump shaft failure, bought a kit from HQ Motor Service, not sure if that was the initial mistake, Kit came with a belt that was 1.5" short, returned it and found a Gates 5040355 (K040355).
Ran unit for approx 30 minutes, outer rib of belt came off.
Checked pully alignment, seems fine, any ideas or known issues with replacement pumps or accesory belts?

That's odd!
Usually a rib tearing off a belt is due to misalignment or even the incorrect width belt, but it would be pretty obvious if you had a belt that was too wide!
After the rib tore off was the rest of the belt in the correct position with 1 empty rib on the damaged side, or by chance did the belt jump over 1 groove towards the damaged side, leaving the empty groove on the "Good" side of the belt?

I wll double check alignment, the outer rib just came off, it kept running after that.
I did check fit on all 3 pullies, seemed ok.
verified belt width with MIL part number and Gates.

Thanks for the input!

Belts don't jump off or shear unless there is a physical issue of alignment. You're on the right track.

Did you replace the water pump? I wonder if you have a pulley shaft that's out of round, and its causing the belt to walk? Under any other scenario, I don't know how it would walk like that if the pulleys are aligned. Can you get a straight edge in the machine and confirm both pulley faces are lined up?

I'd be looking at the alternator too,

Being adjustable, it's the most mobile piece in the trio of pulleys. If it's canted just a couple of degrees, it could send the belt off course.