That sounds very suspicious to me. Mine sat on the lot for maybe 8 months before I bought it. It’s my weekend driver and sits often for a few weeks between drives. In almost 8 months I have maybe 4,200 miles on it and I never get that ESS charging error.
I would think that you have if not a problem with one or both batteries then you have a charging system problem. I don’t think the system is bad at charging the batteries if it’s working properly.
User @Jebiruph is the resident expert here on the ESS system it seems, maybe solicit his opinion.
He has chimed in on this thread several times. Its not an error I am getting the system is just charging the batteries. If I drive enough the start/stop engages. I have chalked it up to sitting for extended periods and when I do drive it's often for only 15min.
Not sure what else the dealer can do besides load test the batteries and scan for codes. But using a trickle charger has solved my issue completely.