Upon running tests on a stand-alone phpBB forum it turns out avatars get deleted even if they are not uploaded avatars just like in PHP-Nuke, the avatar type does not get detected by the forum's profile. In the case of PHP-Nuke even while nothing in it causes the avatars to get deleted the avatar type does not get set when a user's info is saved to the database and this affects how the forum files categorize a user's avatar. The only avatars that users were meant to be able to delete are uploaded ones, the reason why avatars get deleted is because usercp_register.php does not check if an avatar was uploaded before deleting it when a user selects an avatar from a gallery or uses a remote avatar, any avatar regardless of its type gets deleted. For a fix for Nuke not setting the avatar type and the fix for the forums deleting the avatar go here.