Index funds are supposed to cut out the human-driven craziness that periodically infects markets, but the recent meme-stock fever proved the $11 trillion industry is far from immune. The remarkable surge in shares of AMC Entertainment Holdings Inc. and a handful of other stocks is showing up in multiple exchange-traded funds, skewing portfolios, altering risk profiles and exerting outsized influence on prices. Take the $68 billion iShares Russell 2000 ETF (ticker IWM). In the past week through Thursday, AMC powered 70% of the product’s advance. The stock was responsible for less than a 10th of the fund’s return in the previous week.