Why does adding #element_validate give warning about invalid callback?

I have code that adds #element_validate to all fields of a certain type (Link fields).

$form[$field_name]['widget'][0]['#element_validate'][] = '_fix_link_field_value';
array_unshift($form[$field_name]['widget'][0]['uri']['#element_validate'][0], '_fix_link_field_value');

This works fine, but I get this warning in the logs every time an entity with this field type is saved.

Warning: call_user_func_array() expects parameter 1 to be a valid callback, array must have exactly two members in Drupal\Core\Form\FormValidator->doValidateForm() (line 282 of E:\www\Noah\pre\web\core\lib\Drupal\Core\Form\FormValidator.php).

I suspect this is because the existing validator has the format [class, method]. Is there any way to mix these validator formats?

#element_validate expects an array of validation callbacks.

A callback can either be the function name of a globally accessible function, or an array of a class name and method name.

Now I am supposing that '_fix_link_field_value' is the name of a globally available function defined in your module.

So for using it as a validation callback, there is no array required here.

While the first line of your code is adding this callback to the callbacks of #element_validate, the second line is prepending it to an existing callback array, resulting in the following '#element_validate' value:

[[
  '_fix_link_field_value',
  (class),
  (method),
]]

Which won't work. You'd need:

[
  '_fix_link_field_value',
  [
    (class),
    (method),
  ]
]

So what you really want to do is to remove the trailing [0] from your array_unshift call:

if (!empty($form[$field_name]['widget'][0]['uri']['#element_validate']) && is_array($form[$field_name]['widget'][0]['uri']['#element_validate'])) {
  array_unshift($form[$field_name]['widget'][0]['uri']['#element_validate'], '_fix_link_field_value');
}
else {
  $form[$field_name]['widget'][0]['uri']['#element_validate'] = [
    '_fix_link_field_value',
  ];
}