Same thing happened to me. I checked in for tomorrow’s flights. The boarding passes on the app didn’t show my Pre Check. However I downloaded it to my Apple Wallet and the Precheck showed. I called the 1K desk to get ann explanation. They played around and couldn’t figure out what happened. The Precheck on that was on my wallet no longer is visible. I was advised to go to the ticket counter and they would add it to my boarding pass.