Another thing that I will do anyway is to drown the battery to 0% (drive it until limb mode) and then charge it to 100% while measuring the kWh's charged with a kill-a-watt meter.
Anticipating the feedback from the dealer / Zero, I charged the DS all the way to 100% yesterday and then drove to 0% SoC (turned out to be 2% in the end, after it stood for 5 minutes in the garage). In the end I drove 145km, part of which was freeway and many 80+ km/h stretches. And then charged it with a kilowatt-hour meter in between, to see how many kWh really went into the DS. I was pleasantly surprised.
When I went to charge the bike, the app indicated 97v and 2% SoC. So there was still some left in the battery. I then used 2 Delta Qs to charge the bike to 100%. Finally 11.18 kWh (after ~ 6 hours) was measured by the kill-a-watt meter, then the app indicated 116v / 100% SoC and switched to cell balancing. Since the Delta Q charges have an efficiency of ~90% (which is fairly normal with AC-DC conversion, 5-10% loss) and I actually measured 11% deviation between what the kill-a-watt meter indicated and what the Zero app indicated, I can say with near certainty that 11.18 * 0.89 = 9.95 kWh was charged in the DS battery. As the DS battery has a nominal capacity of 11.4 kWh, that means the SoH of the DS battery is at least still 87% right now. I think that's very decent, after 5.5 years. However, when I started charging there was still some in the battery. So realistically, the SoH of the DS would be around 90%. =
Next thing is to get the firmware updates to see if that solves the SoC issue. But the more I am reading threads here (and other Zero related social media), the more I suspect that there might be some kind of bug in the recent firmware. At least on my bike.
As soon as I will have any updated, I will post them asap.