I realise the new phone in play sets you thinking it must be the cause. But it’s a fairly common issue on iOS in general at this point across various different phones, iOS versions, and Monzo app versions.
There are a few threads about it now, and this one’s mine.
Concerning that the delete app, restart device, then reinstall app isn’t working for you. Til now it’s been the only thing that can reliably temporarily fix it. In the restart step, try the force reboot as opposed to just turning the phone off and back on.
One thing that also worked for me in the past was to upgrade to the test flight version.