Configure Native Notes.app to Use Monospaced by Default
It’s currently not possible to set new notes to default to monospaced.
I’m not able to reproduce your issue though; if I choose monospaced from the format dropdown then return will keep the monospaced formatting until I manually choose Body formatting again. Check your OS is up to date (I’m on Big Sur 11.3 and iOS 14.5 and don’t have a non-beta install to test).