You don't need to be an expert in the rules and try to determine on your own whether it's a violation of FINRA's rules. FINRA will do that for you.
Even if Robinhood technically disclosed that they might do it, violating a customer's express orders without documenting appropriate reasons for doing so is generally a violation of FINRA's rules. Brokers have lost their licenses for shenanigans like that.
You don't need to be an expert in the rules and try to determine on your own whether it's a violation of FINRA's rules. FINRA will do that for you.
Even if Robinhood technically disclosed that they might do it, violating a customer's express orders without documenting appropriate reasons for doing so is generally a violation of FINRA's rules. Brokers have lost their licenses for shenanigans like that.