I think I've found a bug:
When a WHMCS user registers a new site on WordPress, if that user doesn't already have a WordPress site, the plugin will create a new user.
It does this by taking the WHMCS users email address and uses the portion of the address prior to the '@'.
But I'm encountering a problem with this!
If I register a site having logged into WHMCS as stephen@domainA.com and create a site, I'm issued a WordPress username of 'stephen'. That's all good so far.
But if I then log into WHMCS as stephen@domainB.com and create a second site, the plugin fails.
It appears the plugin fails to spot that it cannot register 'stephen' again (because it's a different email address). I end up with a 'pending' order, which when I look at it, it has a blank username.