Most of the stats sites generally undercount and no one is totally sure how many servers there are because there's no requirement for them to take part in stats (they can if they want, but it's optional).
I really wouldn't rely on graphs that suddenly go down in a straight 90 degree cliff face. People don't behave like this in real life, users and nodes go up or down more gradually than 90 degrees.
A more likely explanation is that part of the stats have been removed from the data source, so we have a narrower view of the Fediverse.
Contrary to the information displayed on the chart above, the-federation.info reports a bump in users but no drop in nodes (instances?) 🤷 https://the-federation.info/platform/21