FIFO and futures trading

Is anyone clued up on FIFO regulations for futures.

One of my Brokers (Interactive Brokers) does not seem to be applying FIFO to intraday trades.

Meaning, if I buy 1 contract at 100, and another contract at 200. I have an average of 2 at 150. If I then sell 1. One would expect that if FIFO were being applied, I'd be left with 1 contract at 200.

But IBKR is not doing this. I'm left with 1 at 150. They say FIFO is mandatory on futures and there's no options to change it.

I have a feeling that they only apply to overnight trades as I've noticed in the past some of my positions magically have their average price adjusted.