I've had this happen as well, though it's not every order. There was one instance where paypal shows the payment completed successfully, but the order status never changed.
Usually, this only happens if payment was made using something like an echeck where the funds haven't cleared yet.
I'm on WC 2.2.4.
I've had 3 sales today, and the first 2 went through as expected, but the 3rd got stuck on pending.
If Paypal is unable to return the callback, this will happen - this can sometimes happen if your server is overloaded.
I just checked the stats on my server, and it's been at 100% since about noon today (the successful sales were before noon).
If you're on shared hosting that has "unlimited" plans, there's really not much you can do to keep track of that or prevent it.