There are loads of reported issues with the search function:
https://community.monzo.com/t/ios-when-searching-it-does-not-search/40151?u=awjdean
https://community.monzo.com/t/ios-location-duplication-in-search/3521?u=awjdean
https://community.monzo.com/t/ios-bill-split-requests-dont-appear-in-search/58439?u=awjdean
https://community.monzo.com/t/android-search-bug-irrelevant-results/50292?u=awjdean
Hopefully the feature gets an overhaul in the future.