Hmm, looking at uc_roles_user_submit(), this may be where the existing roles are being deleted:
<?php
// If a user's role is removed using Drupal, then so is any expiration data.
if (is_array($edit['roles'])) {
foreach ($account->roles as $rid => $role) {
if (!in_array($rid, array_keys($edit['roles'])) && $rid != DRUPAL_AUTHENTICATED_RID) {
uc_roles_delete($account, $rid);
}
}
}
?>The next user might try adding the existing roles to $edit['roles']