Investors are starting to question one of Lucid's biggest growth opportunities.In July, Lucid Group (LCID 0.69%) stock soared from $20 per share to more than$30 per share. Since those highs, however, shares have slowly sunk in value. Today, shares trade below $20 per share.Why is Lucid stock falling? There's one obvious reason.Lucid stock popped after its$300 million deal with UberOn July 17, Lucid Group announced a blockbuster deal with Uber Technologies. The deal called for Uber to invest $300 million i ...