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.
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