Associating an event to a NetSuite record when Event Autopilot is enabled is disabled while in compose mode because the Microsoft API does not yet provide an Event ID. If a user were to save it to a NetSuite record while in compose mode the Event ID would not be known, and Autopilot would then create a duplicate event immediately on saving the event.
If you also need to associate the event with a specific record such as a transaction just open it again after it is saved and you can do so. Remember with Autopliot the event will already be automatically associated with your attendees so the above may not be necessary.
More details about the feature here: Understand How Calendar Sync works!