If a Robinhood trade is being matched with a better price after it routes then Robinhood didn't offer them the best price. It's how Robinhood makes money. This is the heart of their deal with Citadel Securities. And, yes, the SEC is indeed on their case about this. It's how we know about it.

"How Robinhood makes money on customer trades despite making it free" https://www-cnbc-com.cdn.ampproject.org/v/s/www.cnbc.com/amp...