missing canaries more likely?
insufficient tests that dont assert on the right things?
the existence of a test doesnt mean it catches the right thing
based on the description, id bet the COE action item will be to do a migration that enforces units are passed at the billing service level
theres no good reason for the billing service to make up its own units.
Yeah types and APIs that are difficult to misuse are way better than any regression test, canary, or what have you. Systems that are correct by construction always beat "you're holding it wrong".