First off, welcome to TF.
I'm thinking there is just enough friction on the clutch to allow the shaft to turn without any load on the shaft, but not actually any power to the shaft. You might try
1 Make sure the sure the tractor is NOT running, and in a safe location.
2 fully hook the mower up (just like you are ready to mow with it)
3 Make sure no one is near, the PTO is not enguaged, then crank up the tractor.
But there are a lot of people here with more experience than me.
Just BE CAREFUL, and good luck