The Ending Of
Season 3
Fans of the supernatural series "Manifest," were very disappointed when NBC canceled the series on June 14, 2021. Season 3 ended on multiple cliffhangers, but unless "Manifest" finds a new home, it's likely those cliffhangers will remain unresolved.
In the Season 3 finale, there are two big developments, the first is the death of Grace who was stabbed by Angelina on Flight 828. Showrunner Jeff Rake sold "Manifest" to NBC with six seasons already mapped out, so it was always part of the plan that Grace would die at this point in the show.

The other major development in the Season 3 finale is the second sudden disappearance and reappearance of Ben's son Cal. The most obvious explanation is that Ben's meddling with the airplane wreckage caused Cal to age up somehow which implies that there is a divine plan in the plotline that is linked to Flight 828 crashing and reappearing again.

The show didn't have time to explain why 200 people end up getting premonitions or why Cal aged up. But since Season 3 introduced the possibility of an impending armageddon – even if it was just a cultist theory at that point – that would be our best guess as to why Flight 828 disappeared: to give the passengers premonitory powers that they can use to save the world.