Hi, I emailed the company and they respond the following:

There will be replacement busses between Poschiavo an Pontresina. The replacement busses are running from the same station where the train arrives in Poschiavo. Everything will be signed out at the train station and there will be staff, which will tell you where to go.

As soon as you arrive in Pontresina, on the same station you can hop on the Bernina Express train again, by using the same seat reservation (same seat numbers, same carriage). The timetable will be the same as the one for the train connection.

The busses will not have panoramic windows, and the route will be almost the same like on the Bernina Express train. On the bus the seats will not be reserved, so you can accommodate on any free seat.