While having similar external dimensions, the iPad Pro 11" is fractionally thinner than the iPad Air5. As such, a fitted case designed for either may only fit the model for which it is intended. Many third-party manufacturers cases are designed to fit both the Air5 and the Pro 11".
Here are the published dimensions; you'll note the the depth differs by only 0.2mm:
However, all models of iPad Air 11" have exactly the same physical characteristics - with only one exception. The second through fourth generation iPad Pro 11" have a larger camera array than the first generation iPad Pro 11"
Due to the difference in camera design, a fitted case designed for the first generation iPad Pro 11" won't fit the second generation or later - but cases designed for the second/third/fourth generation are compatible with all of the 11" models.
As such, armed with this knowledge, finding a suitable case for your iPad Pro 11" should be straightforward - as there are many from which to choose. Perhaps consider STM or Otterbox.
https://stmgoods.com
https://otterbox.com