A registration field never wired up on mobile
The mobile register screen never sent terms_accepted, so every mobile signup failed validation with 'terms_accepted must be a boolean value'.
Built, and nothing used it
Shipped, typechecked, dependency-injected, unreachable. Nothing ever called it.
The backend has required terms_accepted since the beginning. Web has a checkbox, gates submit on it, links out to Terms and Privacy. Mobile's register screen had no checkbox, no state, no key in the payload. Every mobile registration failed the same validation, always, with a message ("terms_accepted must be a boolean value") that gives no hint the real problem is a missing checkbox in a different app.
Not subtle — a fully built feature on one platform that was never built on the other. Obvious the moment you try to register through the app you're supposedly testing, invisible if you only read code or run tests written after the fact by someone who already knew the flow worked. Parity has to be walked on the platform in question.