I just got "Your Quick Feet buff has worn out, back to normal dancing time!" I think there was a buff that shortens time between Trick or Treating so maybe that's what was supposed to happen? I didn't notice if I actually had any buff before getting that message.

No, there's no buff for shortened ToT time. The timer should be shortened near the end of the event, such as during the evening of the last day (Halloween).

It's most likely caused by the shop discount buff expiring. There's a thread dedicated to it here: https://subeta.net/forums.php/read/904411/-Your-Quick-Feet-buff-has-worn-out-back-to-normal-dancing-time