Fortnite's drastically diminished Save the World XP rewards was a bug, Epic says, but now it's fixed.
Epic Games says a recent change to Fortnite Save the World that drastically cut back on the XP players could earn was in fact a bug, and the issue is being fixed.

The trouble came to light yesterday when players discovered that Epic had apparently made a change to the XP cap on Save the World, the original Fortnite PvE experience. Fortnite Save the World is pretty much an afterthought at this point, long ago left in the dust of Fortnite Battle Royale, but players can still use it to earn progress in Fortnite overall. As Forbes explains, players could previously earn multiple levels per day from playing Save the World, but with the start of Chapter 6 Season 1, it was cut to a total of nine levels per week, a drastic reduction.