How can I locate unauthorised Apple Pay transactions on my iPad?

If a credit card has been compromised, you will see those charges in the Wallet app. It does not mean that the charge occurred from your device or even with your Apple ID. Many scammers will use the description of the charge to make it look like it came from a well known company so you will not dispute the charge. Credit Cards are commonly compromised when used on a terminal that has a card skimmer. Many of those are found at gas stations where the thief can install them uninterrupted overnight.

Yes, notifications can go to different devices depending on Focus modes and other factors.

The determining factor is not notifications. When a transaction is made, the card is either >Card Present< or >Card Not Present.< Apple Pay transactions completed via NFC (Tap-To-Pay) are considered Card Present. Online transactions are considered Card Not Present. iPads do not have NFC for Apple Pay, so online iPad use is always Card Not Present transactions.

Ask the bank if the fraudulent transaction were card present or card not present.

There is a Fetch Data option on your card in the Wallet app that can be turned off so that information will not show. I do not have my card on multiple devices, so I am unable to test if that setting would be the reason why it shows on one device and not the other. Here is a Support Article that gives more information about the Fetch Data option and how to turn it on/off. Are you using your other transactions appear on both devices?

View your account balance and full transaction history in the Wallet app - Apple Support

If the charges are connected to a compromised credit card and bank account, please contact the bank that issued the compromised credit card. You should also contact your local police.