Using setError with a custom resolver

[stefan-girlich]

I am using zodResolver from @hookform/resolvers/zod with RHF on the frontend and in my API middleware.
I am applying a duplicate/uniqueness check on a field that may pass in the frontend, but fail in the middleware if a conflicting entity with has been created after page load. tldr; you enter "My unique name" in the FE, pass validation, but in the middleware we find out "My unique name" has already been used.

I am returning the corresponding validation error (ZodIssue) as an API response and I want to use setError to highlight the error as if it had been detected before submission.

The docs state that validation will still pass if the local check is passed, making it sound like setError rather acts like setErrorMessage.
Calling setError('my_field', { message: 'Error!', type: 'custom' }) after submission (and thus, after server validation) indeed does not have any effect. In fact, even calling setError('root.foo', { type: 'any error' }) does not update errors returned from useForm.

What am I doing wrong?