Front running order flow is also done on Limit orders. They only fill your limit order when HFT has a fill elsewhere for cheaper to pay for the commission to RH ($0.52 per trade according to their disclosure documents) and the fee for the HFT frontrunner.
Screwing over customers like it's 1999, just like with the specialists at the NYSE and in the trading pits in the old days, today, the front is now done with HFT.
I swear they hold my options orders for minutes before filling me even though I hit the bid. Happens more frequently than before. Disgusting. I have an entire thread on that.