filter 'get_avatar'

Hi

In my site, users can upload their own avatar from their profile. It create a user_meta called “avatar” where the url is stored.

I’d like to display this avatar on my site, and if the user doesn’t have upload an avatar, display the gravatar.

I try to use a filter to do it, but it doesn’t work. Here is my code. I’ve modified code found in internet

function be_gravatar_filter($avatar, $id_or_email, $size, $default, $alt) {
	$custom_avatar = get_the_author_meta('avatar');
	if ($custom_avatar)
		$return = '<img src="https://freeletics-stats.com/'.$custom_avatar.'" width="'.$size.'" height="'.$size.'" alt="'.$alt.'" />';
	elseif ($avatar)
		$return = $avatar;
	else
		$return = '<img src="'.$default.'" width="'.$size.'" height="'.$size.'" alt="'.$alt.'" />';

	return $return;
}
add_filter('get_avatar', 'be_gravatar_filter', 10, 5);

I don’t understand how the parametres are set. With that code, all users on comments has the same avatar : mine.

Any ideas? Thanks