Thanks, all. I did talk to a Magnum tech today and he amplified what I learned from Sunchaser. He said the unit is programmed to "rest" for 4 hours periodically after the bank reaches full charge as determined by voltage, not the SOC percentage shown on the screen. So, unless this resting feature is disabled, the unit goes into standby if the battery bank reaches its target voltage, regardless of how the system has calculated SOC.
Also, the blinking charge light on the remote monitor, indicating that the charger is on standby, cannot be over-ridden by the on/off switch. It will continue to blink until the unit starts charging again, or until AC power is removed.
So, bottom line seems to be that my charger is functioning normally.