According to the following from the event webpage: "This boost will net operatives increased EXP gain as well as an increased rare item drop rate for the duration of their current instance of Trick or Treat! So before heading to the Quest Counter, check to see if you have any of the items in this article!"
The PSE boost is supposed to last for the remainder of the quest, the PSE boost does activate more the more halloween clothing you wear, but the PSE boost expires not even 5 seconds after activating or leveling up. Is this intentional, or is it actually still active despite the buff icon at the top-right of the screen disappearing.