The <100% SOC has been discussed on the forum several times. Search for BMS and IBS.
It's a carryover with the BMW engines and electrical system. The Grenadier (and presumably the Quartermaster) has a Battery Management System (BMS) with an Intelligent Battery Sensor (IBS) mounted on the negative post of the main battery. The IBS monitors all current flows in and out of the battery. The BMS and IBS manages the smart alternator output during vehicle operation to maintain the battery SOC at around 80% with some variation. The goal of the smart alternator + BMS + IBS is to ensure there is always enough battery charge to meet the electrical demands of the vehicle including stop-start, without unnecessary charging of the battery to ~100% SOC which increases alternator load and fuel consumption, and decreases battery life. This strategy is primarily to increase fuel efficiency.
The other purpose of keeping some SOC in reserve is for regenerative charging during overrun (closed throttle, deceleration, braking).
To see this in operation observe the trip computer instant fuel reading and the off-road electrical screen charging rate. During overrun the fuel consumption drops to zero and the charging rate rises because the alternator output is temporarily increased to send 'free' current to the battery without using fuel. BMW has done this for years. It's not unique to Ineos vehicles.
When charging the battery it's essential that you don't connect the charger lead directly to the negative battery post. Positive is ok, negative is not. Connect the negative to a body earth to ensure the IBS is in-circuit and can sense the charging current. If the IBS is unaware that the battery has been charged the displayed SOC will be wrong. I assume this self-resolves over time but the BMS and SOC will be messed up in the interim.
More on the IBS in
@DCPU's post
here and battery charging in
@Xrford's post
here (and many other threads).
You can read more about BMW's intelligent alternator control in
@NQ94's post
here.