Would be great if it is just the gas...sucks for you guys stuck with 91...limits what you can get out of the car you paid for! Anyway, let us know how it goes...really hope you can find a reliable solution so you can enjoy your car!
I mean this isn't my wheelhouse, but is it not worth considering the fuel quality could have changed? I would personally ride out my tank and try another known good station to see if anything changes before I go digging into an issue no-one else has seen here before.
Either way, shoot BT an email in case, you never know how long those support queries can take.