You can view all the tasks related to a reservation from the Multi-Calendar, the Inbox, or from the reservation page itself. In the Multi-Calendar or Inbox, click 
in the right side-bar to view associated tasks.
The widget shows not only the tasks associated with the reservation, but also any tasks scheduled during the guest's stay, one day before check-in, and one day after check-out. Therefore, if you have consecutive reservations, you may see tasks related to one reservation in the widget of the next reservation.
In the example above, the "Post check-out cleaning" task appear twice: Once for the previous reservation and once for the selected reservation.
Likewise, if you have tasks related to a reservation that are scheduled more than one day before check-in or more than one day after check-out, they will not be displayed in this widget for the associated reservation. Instead, they will appear in the widget for a reservation whose dates align with the timing of the task.