Dec 17th is a Sunday, and Trenitalia (or more precisely RFI) often plans engineering works in the weekend. That leads to changes in schedules and/or bus replacements.
Trenitalia also has the habit of often finalising the schedule, and opening bookings for that route only at the last moment. Last summer relatives visited me, coming from Milan, and were only able to get tickets a few days before their actual travel date.
And in an attempt to increase the confusion amongst train travellers even further the SBB website has the habit of throwing up absolutely useless error messages in such cases…
SBB and Trenitalia need to find out a way to handle this better, or the default for people going from Milan to Switzerland will be to rent a car.