There was a recent report of an incorrect quantity of payments being displayed, I’ll see if I can find it…
EDIT: Found it. Although this issue was the incorrect quantity of payments made to a vendor - and it was a display issue which was fixed with the usual uninstall/reinstall dance;
Maybe try the usual?;
Uninstall the Monzo app
Restart your device
Download & install the Monzo app from the App/Play store
Run the Monzo app & login (using the same email address you used to initially sign up to Monzo)