On iOS push notifications are dispatched to the app if it’s open, which presumably just refreshes the feed behind the scenes. If you send a payment and exit the app instantly after, you’d see the push notification.
My wife was, and is a monzo customer. I’m sending it to monzo contact rather than entering bank details.
It would explain why I thought it worked before though, it probably did, it’s just we’ve both become monzo CA holders, so it’s been swapped to a faster payment in the background now I’m presuming, introducing the bug to the payment process.