Also wondering the same thing. I'm booked for LHR-MAD on IB in mid June 2023, hoping to use one of the OW lounges in T3 rather than the crowded BA ones in T5, so I'm hoping IB stay at T3!

Both BA and IB MMB don't list a LHR terminal at the moment, so I'm wondering if this is a decision that is in the process of being made? Then again when I type the flight number into the LHR website it lists all MAD flights (BA & IB) as T5 for June??