When I have one of these mystery problems , I grab the Go-Pro knock off camera , clamp it to the mower frame then go mow or in your case turn the PTO on & off till the belt comes off
then pop the memory card in the computer & play it back full speed to find where the belt jumped off , then rewind just that section and go through it a 1/2 speed
Don't be tempted to do it in single frame mode as 15 minutes in single frame takes about 10 hours to step through.
Some times I need to make a couple of videos if it looks like the problem was out of frame the first time.
Of course an assistant can be used to man the mower & in that case just be careful around the discharge side of the deck .