6.0 FICM volts drop only when running

I've got a 2005 F550 6.0 that starts and runs ok, but has low FICM voltage. I've tried three different FICM's as well as another FICM relay, but none of these change how the FICM voltage behaves... If I turn the key on, FICM voltage starts out low, then slowly climbs to 48 Volts. If I start the engine, FICM voltage immediately drops and stays low, somewhere below 40V.

Batteries are good, alternator is good, tried 3 different FICMs, tried a new FICM relay. I am using AutoEnginuity to monitor FICM voltage which I know works because I do this routinely on lots of 6.0 trucks.

Any ideas what might cause this behaviour? I'm very doubtful that it's actually the FICM itself.