Wallet transactions disappear

It is only possible to see the last 10 transactions in Wallet. If you want details beyond that, then you need to use your bank's app.

It is not possible to delete individual transactions, but as you use your card, the transactions will go further down the list, and will disappear from the Apple Wallet.

If you want to delete the entire list of transactions, then you need to delete your card from Apple Pay and set it up again. Once the card is installed again, your transaction list will be cleared.

Apple Pay setup:

Set up Apple Pay - Apple Support

You're welcome.

Yes, it could be useful with the ability to save all Apple Pay transactions, and not just 10. I do not know why it is decided it should be like that.

You can always send Apple a Feedback and suggest the change.

Feedback - Apple Pay - Apple