I suspect the data artifact is related to the transition from 0.18 to 0.19 and something changed in the way active users was counted in between.
My understanding is they started counting votes as activity.
The difference between the peak and the plateau is higher on lemmy.world and lemmy.ml -- I suspect this is because they were more popular places to sign up during the protest.
Makes sense that people who care enough to look into smaller instances would be more committed.
Latest Batman villain: The Interrobanger