It doesn’t change the original question much, but there are actually quite a few well documented cases of fertile mules. This page lists some older examples going back to the 19th century; some more recent (peer-reviewed) cases are listed in

• A fertile mule and hinny in China
• Key Aspects of Donkey and Mule Reproduction
• Fertile mule in China and her unusual foal
• Fertile mules
• A foal from a mule in Morocco

With dozens of instances being documented in spite of farmers actively trying to prevent mules and hinnies from breeding, it's likely that a fairly significant percentage (though of course a minority) of them are fertile.