One of the reasons to avoid "internalization" in equities is a trader may loose the payment for adding liquidity.
In futures, I believe, internalized orders are illegal in the US.
Edit: You may occasionally get "improved" price on a limit order on an exchange. It's not evideance of "internalization" on its own.
However, in your example you bid above the best ask price and was filled at best ask. This is how an exchange normally operates and it is not either "price improvement" or result of order "internalization". Bids substantially above best ask are filled like "market" orders.
Second edit: If you wanted the buy order to be only triggered if the price rises to 871.50, you should have used market if touched or limit of touched type of order. These are effectively stop-market and stop-limit orders except you use them to enter a position and not to close and existing position.