I have noticed this for a long while now but never brought it up. Just recently Taiwan had an event and just like other regional events in the past all gyms near me had pokémon in them that lasted for days with the exception of maybe one or 2 coming back but notin waves. I mean it was great. I got gold in some. As soon as the event ended my pokémon started coming back like crazy. It was like a light switch was cut on.
I think you can pretty much tell why this happened. GPS hacking. Spoofers port over to exclusive events and play in those areas for the entirety of whatever the event is and when it’s all done they spoof back to their regularly scheduled program. People not bothered by spoofers often ask “what does that have to do with you”? I usually do not answer back or just block them. It’s an augmented reality game. Figure it out.