The BA ORD lounge has always done this.

I've had RJ and QR flights out of ORD. Have gone to use the BA lounge each time (much better then the AF lounge). For the RJ flight, the BA flight had just been called for boarding and they claimed the lounge was about to close, even though there was still over an hour left before the official closing time.
For the QR flight, they tried to send me to the AF lounge. I said the BA lounge was much better and pointed out the QR J boarding pass and QF Emerald status. When they noticed that the QR flight boarded before the bulk of the BA pax would arrive, they let me in.