This is troublesome, because once you complete a challenge, you're unable to repeat it.

Users that complete this challenge lock themselves out of the best idols/hour in the game. Not good!

For now, I'd advise users to intentionally reset the challenge before reaching zone 250, so you may continue grinding it for the best idols/hour. To developers, I'll request that we're given the option to replay challenges for 0 rubies, or less preferably, rebalance it so freeplay is always better to grind than any challenge... Locking ones self out of doing something optimally is really really disheartening for certain min-max sort of players.