I'd check,

1.the tensioners, make sure the lip is intact on both. and that roll freely.

2.Make sure the front pulley is seating properly on the flat plate deal w/ the big square on it. I'd bet this is your problem, if it doesn't sit flat you'll have too much space and the belt could get let loose.

3. Make sure your using the 20t front pulley, at least for this experiment.

The tensioner should effectively pin the belt in place. There's not enough room for the belt to get between them, normally.

I'd take off, the all the outside/battery side,belt covers.

Check to see if the front pulley is seated properly.

Tighten clicker nut all the way.

Set the tensioners to the stock settings. then run it in the drive or street and see if you can see the problem happening. Or at least where it's starting.

Or on the bench, sounds like it could happen there to.

I bet your front pulley is seated, it's not easy to see and depending which end you put the belt on last can be hard to get it clocked properly.