get_terms() returns an empty array when called from delete_user action hook

When calling:

get_terms('post_tag', array('hide_empty' => 0));

from action hooks:

delete_user
deleted_user
wpmu_delete_user

What’s being returned is an empty array. When calling the same code from different action hooks, a proper non-empty array is being returned. Happens with post_tag, category and any custom taxonomy regardless of whether or not terms are empty or associated to posts.

Original code:

function get_terms_test($user_id) {
	// get terms
	$terms = get_terms('post_tag', array('hide_empty' => 0)); // also tried other non-empty taxonomies
	echo var_dump($terms); // also tried count();
	die();
}
add_action('delete_user', 'get_terms_test'); // also tried the other hooks mentioned

Any ideas why?