I just did a test and everything is working as expected using this code
add_filter('acf/validate_value/key=field_64ba92e423e65', 'validate_email_field', 20, 4);
function validate_email_field( $valid, $value, $field, $input ) {
if (!$valid) {
return $valid;
}
if( email_exists( $value ) ) {
$valid = 'email exists';
}
return $valid;
}
You must have something else that is interfering, as I said in my original response, something that is breaking the ajax response. Either another plugin or some code in the theme.