This is actually expected behaviour - incoming bank transfers affect the Spent Today total.
You’re not the only one to be confused by this though:
https://community.monzo.com/t/android-ios-spent-today-incorrect/52740?u=awjdean
https://community.monzo.com/t/ios-problem-with-spent-today/63164?u=awjdean
Might be worth one of the @Coral-Crew merging these reports.