@Rainwater01 - you nailed it. There's an imbalance. The tech from my installer explained: The system only measures state of charge on the first and last battery module in the series of 6 modules (actually, I was a little unclear on this part of the his explanation). Both of those are 'full' when the other four modules are not, so the system stops charging prematurely. He said that he needed jumpers to get around the fully-charged modules and allow the other 4 cells to charge, but he didn't have those with him. He has to get them from Generac. I was surprised at this.
This seems like a fundamentally poor system design, IMO. Maybe it's an issue with a series configuration -- the controller can't directly balance all of the batteries (?). If they were separately connected (in parallel), this might be possible, but then you only have about 47 volts to work with, I guess.
Do other battery brands have a similar problem?
I wonder if my SOC limit (95%) is causing a problem that Generac hasn't seen before. As installed, they set that limit at 100%. I set the lower limit in an effort to extend the life of the battery, but if it's causing a problem, I would not mind going back to 100%. I don't want to change anything right now, since I'm in the middle of this effort to resolve this issue. But it's something to consider for later, I guess.