“It apparently varies by operator.” (#9)
More likely that the system design / certification / manufacturer's intent specifies the use / display of the FD.
E.g. there is little point in displaying a lower reliability 'duplex’ FD system when flying with a highly reliable ‘triplex’ autopilot.
By not showing the FD, this removes any possible confusion / clutter during the approach and forces the crew to focus on the raw ILS display for monitoring the accuracy / progression of the flight path.