If you mean you create a user in Joomla, say "One Two Three Four" and that user appears in Moodle as First Name: One and Last Name: Two Three Four. This is correct and not an error.
Joomla only has one field for all parts of the name. Thus, the first space is used to break up the first name and the last name. Some names have two names, some three, some four, some five and some only one name. Therefore, it is not possible to know which is correct. This is similar to how Hikashop and other extensions work.
To overcome this, most people will use a user profile extension to capture different first names and last names when registering in Joomla such as EasyProfile. You may also use a Social extension such as EasySocial or a ecommerce extension like Hikashop or VM. These will all allow you to have multiple fields for the name during registration. This will then be passed to Moodle as you require. If none of these extensions appeal you can even use Joomla's inbuilt user profile plugin to achieve the same.
