We appreciate you clarifying this for us, x_eddy,
The passes may be remove due to iCloud account syncing. You can test this by disabling your Wallet temporarily in iCloud. You can follow the steps below to test this behavior to determine if this is what's causing it.
1. Go to Settings > [Your Name] > iCloud > Wallet to disable Wallet
2. Add the missing passes to your Wallet app again.
3. After 24-hours has passed, check your passes to determine if they're still there.
4. If your passes are available, you can enable Wallet in iCloud again in Settings > [Your Name] > iCloud > Wallet.
If you're still experiencing the same behavior, we'd suggest that you reach out to Apple directly to investigate this further with you. You can reach them here: Contact Apple Support
We hope this helps.
Take care!