I think Altea system SK now uses automatically defaults to check the baggage through even for overnight connections. From experience - if agent checks you through and then offloads (as e.g. decides an overnight bag check through cannot be done) you lose your UA seat assignment. It happened to me twice on LO-to-UA overnight connection via AMS (I went to united.com and reinstated it) - seems like Altea and SHARES have some communication problem.