The event just ended for Big Thunder, I ended up in #32 spot with just over 500 points. I received absolutely nothing.
If you still haven’t received rewards, reloaded the game. They have been awarded already.
The earlier you join, the sooner you get them AFAIK.
I always figured the delay was grace time for people in your bracket still in a match.
No, there is no grace period. If you’re lucky you might slip a match end in after the end, but you’re racing the database query. IceIX has said so.
If you’re in a match when the tourney ends, you don’t get credit for that match. Happened to me a couple of times where I exit the match only to be back on the Event screen. Today, I squeaked in at the <1 mark, jumped me from #26 to #22. 
To the OP, I’ve always gotten my awards within a minute or two of the event end.