Step 1: Get Hat Pikachu as a quest reward.
Step 2: Complete the quest.
Step 3: Press claim reward, and run.
Step 4: Wait
Step 5: After the event has finished, catch the Pikachu. It should still have its hat and sunglasses.
I also tried this on Squirtle and it didn’t work. So I’m not 100% sure.
Thats not a glitch, just an old pokemon you saved up.
That’s not a glitch, as long as you completed the quest before an event ended, you can still get the Pokémon Even after event. I was still able to get my sunglasses Squirtle from research award couple days ago, long after Squirtle CD last month
it as if you didnt have any pokeballs so you can go to a stop then catch the task pokemon without losing your chance
Do you need to escape from the encounter screen and then spin pokestops get balls?
If yes, better not feed any berry to that Pokémon beforehand
Yep.
This berry glitch has been fixed
Thanks for the INFO 
