Age of Undeath reads:
…the Abomination does not start to decay until you control 6 or more [Skeletons or Skeletal Mages] at the same time.
If you hit the threshold that causes the decay to begin, but then drop below it, the Abomination continues to have health decay until it dies or is resummoned.
This may not actually be a bug (and is deliberate to stop people from doing exactly what I wanted to do), but I think it’s a little disappointing.
My intent when I discovered this was to dance around the threshold, trying to play with an up/down balance, to keep an Abomb going in a Mage Sacrifice based build.