If you use the Meetup feed and events disappear, this is the issue on their side, or the way in which their export works.
 
You can try to preserve the events using the two options:

  1. When you add a feed URL, select “On refresh, preserve previously imported events that are missing from the feed” option.
  2. When you edit the event or make some changes, check “Preserve changes made to events retrieved from feeds”(“Publish” box on the right), this will save all changes no matter if you import the same event again.

If the above options do not work, please try this workaround: add Meetup feed, then delete the feed and select “Keep Events” when prompted. You won’t get the updates of events, but you could set a task for yourself say the 1st of the month to reset your Meetup feeds – pull them in, delete the feed and keep the events.