Two things to try depending on which of these yours is closest to:
The fix for me was to upgrade to the TestFlight build. You can join the TestFlight program and update here:
If your issue is more like this one:
The most reliable fix I’ve had for this is to force restart your iPhone. Instructions for that vary by iPhone model and you can find those here:
Alternatively, swap to Android. These such bugs are not present on Android, and Monzo shows no signs of attempting to fix bugs or improve performance on iOS. Android is much better supported by Monzo unfortunately. The iOS app has been suffering from a multitude of unfixed bugs and performance issues for years now, in addition to lacking some features and polish.