Discovered this morning that one of my two MPPT controllers, in this case the 100/50, had voltage from the PV array and showed the correct battery voltage, but refused to supply any current to the batteries despite the batteries being only about 50% charged.
The system is controlled by a Cerbo GX and all components are connected via appropriate communication cables. Over a year ago, when I installed everything, I enabled local Victron networking despite having the communication cables because...why not.
What corrected the issue with the MPPT 100/50 seeing good voltages but producing no current was to leave the local Victron network and rely exclusive on the communication cables. It appears the local network had somehow decided to shut off the current, possibly due because it thought there was an issue with battery temperature -- and there is no battery temperature sensor in my system.