I'm unsure if we can resolve this issue.
An error should be associated with a field, aligning with how react-hook-form and resolvers function.

When you configure a custom error message using AJV (ajv-error) for the required property, AJV doesn't substitute the default error message with your customized one. Instead, it introduces a new error at the top level.
Due to this behavior, we can't establish a connection between the custom error message and the default one, which is why you're seeing "": {}.

After delving into this, I haven't discovered a way to substitute the default error message with the customized version. If you're aware of a solution, kindly share it, and I'll attempt to update the resolver accordingly.

This CodeSandbox shows a basic AJV validation with a custom error message
https://codesandbox.io/s/suspicious-silence-kt7qqk

The result from the validation:

[
    {
        "instancePath": "",
        "schemaPath": "#/errorMessage",
        "keyword": "errorMessage",
        "params": {
            "errors": [
                {
                    "instancePath": "",
                    "schemaPath": "#/required",
                    "keyword": "required",
                    "params": {
                        "missingProperty": "username"
                    },
                    "message": "must have required property 'username'",
                    "emUsed": true
                }
            ]
        },
        "message": "username is required" // ⚠️ we can't associate this error message with the error
    }
]

@jorisre Can you explain this more to me? I'm trying to fix it

Me too

I just encountered this issue, not being able to display a custom error message for a required property.

What I don't get is that ajvResolver already handles the case of a required error that is bind to the root and not to the path of the field.
In https://github.com/react-hook-form/resolvers/blob/master/ajv/src/ajv.ts, you will find it from line 11 to 16.

 // Ajv will return empty instancePath when require error
  ajvErrors.forEach((error) => {
    if (error.keyword === 'required') {
      error.instancePath += '/' + error.params.missingProperty;
    }
  });

This piece of code rebuilds the path from the missing property in the case of a required error, so it feels like ajvResolver will handle the case, but actually, it does not match the payload of the actual error (which is the format mentioned in #615 (comment).

ajvResolver's code is quite standalone and easy to fork, it may be a dirty quick fix (not having every react-hook-form/resolver points in mind), but adding the following loop after the first one did the trick for me:

ajvErrors.forEach((error) => {
    if (error.keyword === 'errorMessage') {
      const subError = error.params.errors[0];
      if (subError.keyword === 'required') {
        error.instancePath += '/' + subError.params.missingProperty;
      }
    }
  });

As stated, it may be simplistic and/or dirty, but it definitely feels like ajvResolver could handle this.
I hope this will help!