What happened with this one was that after fixing the bug that caused level 60 NPCs to spawn on lower levels' maps, they ran a script that cleared the NPCs off most players' or all players' maps. It seems like they did this regardless of level.
You all probably know that NPC bases become tougher as you beat more of them. A side effect of the script is that many players have had their NPC difficulty level reset to something lower. A couple players on Reddit or Discord reported major reductions. However, the effect seems variable: my main account has seen no change, and my alt has seen the difficulty level decline just a bit (to level 59 NPCs, roughly the same resource rewards as before).
Side question, I am pretty sure that a very similar bug has occurred before, at least a year ago. Does anyone remember this?