To prevent your VaxYes pass from disappearing, you can disable the "Automatic Updates" on your VaxYes wallet pass.
If your VaxYes pass is currently missing from your Apple Wallet, please add it to your Wallet again.
Once your VaxYes pass is in your Apple Wallet, visit your Apple Wallet app from your iPhone home screen and locate your VaxYes pass.
Tap the button that looks like three dots in a circle. It should be in the top right corner and it will look like this:
Click "Pass Details":
Toggle the "Automatic Updates" to the off position.