@4junk3000 In the old major events, you had to finish each act before the next started and you had to finish the event before the end time or it would disappear.
When they began the Multi event, which is more like 4 mini events put together with no crafting etc, you had the entire event to finish any of the acts and it would let you finish whichever part of the event acts you were on even after the event ended. So if you started part 3 of all 4 acts it would let you finish and get the third prize of all four acts and then the rest would disappear.
The game even tells you “___________________ Event is over! But don’t worry! You can still finish your current quest and earn its reward.”.
Looks like this one was supposed to work that way but it is not working properly.
However. I have seen other multi events or mini events where this was to happen and it didn't work.
So I would suggest in the future, people finish by the end time and do not assume you will be able to finish afterward.