In short: it's by design, not a bug.
In long: We cap Domain Power costs at 400, which is the real "hard cap" in the game. You can be "soft-capped" at 100,200,300 Domain XP depending on your Government, but the Age 8 Governments have a cap of 400, which is the highest you can achieve in-game. If players want to keep using powers after they exceed their Domain XP cap, they should upgrade to a newer Government.
Powers that scale in cost are like that are intended to deter possible exploits or balance issues. If the power is good enough that you keep clicking it we increase the cost until you can't click it anymore. If you still like using the power at 400 Domain XP cost it's a sign to us that there's something deeper going on.
Do you feel like pawning Artist themselves are that valuable compared to other domain powers (including Social Fabric) or is it that you have so much income that reaching this point is inevitable?