Assume that airplane was configured with wing TAI "on" at the time that another problem caused the crew to switch "off" one of the engine bleeds.
At no time during the QRH actions to resolve the bleed fault would the airplane be asymmetrically wing anti-iced, i.e. one problem doesn't give you a much bigger problem.
I think this may be the reason for the auto function of the isolation valve being associated with engine bleed switch position.