Ok, if anybody else has this issue, perhaps this will help them.
In program/include/main.inc, after the comment at line 588, add these two lines:
$user_name = substr($user_name, strpos($user_name, "-")+1);
$user_email = substr($user_email, strpos($user_email, "-")+1);
This will work, as long as the "prefix" added by your system does not have a hyphen in the prefix. My system uses the domain's account name, and it does not allow a prefix in the account name, so it's fine for my situation.