Coin changes to 3-leaf clover after playing sometime

Bear in mind that Temple Run 2 is a 'free to play' title - they make their money from people buying in-game items, so there are lots of ways to burn through coins and gems.

In this case you can 'gem-power' a power up so that the next time you use it you get more from the power. 1 gem gives you 1 boosted power up use, 2 gems gives you 2 and so on.

If you're going for a long score run you might cash in 5 gems so that you can use the boosted power 5 times.

To be honest, for the amount you get back it's a waste of gems.

I was frustrated with the same issue. I believe that the lifetime coins challenges do not start counting until after the objective is activated (which could be a bug). You probably activated Gold Miner first, collected nearly 400k coins, and then completed a separate challenge that then activated Midas Touch. From that point all coins collected applied to both Midas Touch and Gold Miner, but because Gold Miner was open first, it has more progress. And, of course, the Midas Touch progress bar is going to fill half as fast as Gold Miner's.

I actually stopped playing after I noticed this. Even though they decreased the +2 Gem Powerup to level 9, it will just take too long to complete the lifetime challenges. If it had been counting my true lifetime progress, they would have been completed already.