OK, updating this question very late - actually solved it very shortly after asking.
This turned out to be a timewaster question. Apologies.
But if anyone should come across a similar problem then please check for any REGEX declarations in the Form fields. For instance, a ng-pattern="/^[a-zA-Z]/"
Yes, this will do what it says, and exclude Kanji. Surprisingly it does NOT then put helpful validation error on the form field so from UI perspective it appears that the foreign language characters simply weren't registered.