[Form][Validator] All index items after children are to be considered grand-children when resolving ViolationPath

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | unsure*
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11458
| License       | MIT
| Doc PR        | -

* BC Breaks: The old behavior had a unit case specifically testing the case of a grand-children form. However, this behavior is not documented anywhere and the fix seems to have no adverse effects on form validation.