Just checked and it's the same. I have to wait for 2.6.1 to get approved first then submit a fix. If you don't want to wait and have FTP access to your server, then download /applications/invite/extensions/core/MemberSync/Invite.php and find:
\IPS\Db::i()->update( 'invite_members', array( 'is_member_id' => $member->member_id ), array( 'is_member_id=?', $member2->member_id ) );
Replace to:
\IPS\Db::i()->update( 'invite_members', array( 'is_member_id' => $member->member_id ), array( 'is_member_id=?', $member2->member_id ), array(), NULL, \IPS\Db::IGNORE );
Save and upload.