While the Hatchaton event was going on, we also simultaneously had a Let's GO Pikachu & Eevee event tied into it which saw an increase of many gen 1 Pokémon and Pokémon like Slugma, Gulpin and Anorith too. With this event we received a new Special Research Quest called "Let's GO, Meltan". Professor Willow and Professor Oak were discovering more information about Meltan and required our help in their research. They were getting closer and closer to discovering more about Meltan.
This is also why we saw an increased spawn of all these Pokémon, as several generation 1 and other Pokémon such as Magnemite, Exeggcute, Slugma and Gulpin were featured as Pokémon required to capture or evolve during the quest. Now this quest is still available and won't just disappear if you didn't complete it yet. Just some of the Pokémon will be much harder to find again now for the various steps in this quest as their spawn rate has been set back now.
I managed to finish my Let's GO, Meltan quest on the last day of the event. I finished the final 3 steps on a Lucky Egg after a Raid. As I became Ultra Friends with someone during the Raid. It was close though, just after catching Meltan my Lucky Egg ran out. That's because I had some trouble finding 2 Omanyte or Kabuto once I reached that step. It's all fine though, as I got a nice amount of bonus experience from finishing these steps on a Lucky Egg.
Were you able to finish your Let's GO, Meltan quest during the event?