The belt has burned
You can see several spots where it has been eaten away exposing the fibers
This happens when you do not have enough tension on the belt so the pumps put a load on the belt but it slips on the engine pulley burning the belt
So the first question was the belt correctly routed
and the second one is were the tension springs in the correct place.
I have seen all sorts of silly belt runs because the owners failed to take proper notice of the belt run before it was removed.
Same thing with the tension pulley
I have even seen one with the pulley sprung backwards because he owner forgot where the spring went.
If the belt is jambed then the engine pulley rill rip the notches off the belt one at a time.
Once a few come off, the belt will separate .
As previously noted it is a Stens belt, not a genuine Scag belt.
IT IS SUPPOSEDLY AN OEM SPECIFICATION BELT as it is in the 265 series.
I have never had a problem with Stens belts but I have only fitted them to a Liberty