I figured out why the ACR didn’t flip to combined in time… right out of the documentation:
Blue Sea Systems CL-Series BatteryLink ACR PN7600 Owner’s Manuual
Internal time delay prevents relay action for transient conditions, voltage must
be within range for approximately one minute to cause closure, relay will open
when overvoltage is detected for approximately 15 seconds.
The regulator definitely went from an acceptable charging voltage to an overvoltage situation in less than a minute.