possibly the 48v battery pack or the DC-DC converter?
explanation:
the truck's electrical components that have to do with generating and storing electricity are: 12v battery, MGU, 48V Lithium Battery Pack, and the DC-DC converter.
they've replaced 12v battery (used for starting truck and storing electricity to start truck and run any other things running on 12v - like your engine), the MGU (the generator), software updates (not really have anything to do with your issue). So, we know those 2 out of three components are good. Therefore, at this point, the only thing that hasn't been touched is the 48V battery pack and the DC-DC converter, which is used for the mild hybrid E-Torque system. The 48V is used for storing electricity for start-stop, power various components, help with starting from dead stop, and CHARGING THE 12V BATTERY via a DC-DC convertor.
Since you only mention the voltage drops as you were driving, the only logical options are either the 48V or the DC-DC converter.
48V: energy is not being stored from the MGU. However, there should be a warning or something when a reader is attached to the OBD port.
DC-DC converter: i THINK this is your issue, since there is no warning. When a DC-DC converter fails, energy is not being converted from 48v to 12v to be stored in your 12v battery, therefore, it is depleting the energy in your battery and result in low voltage.
Think of it this way: You have a closed water tank system that you recycle your water as you use it. The 12v battery is the smaller tank with purified water sitting closer to the floor, and feeds your faucet and your fountain; your 48 battery is the reservoir tank with not-so-clean water sitting on your roof of your building, which holds more water and has higher water pressure. As you use the water to wash things from your faucet and water fountain it is depleting the water from the smaller tank closer to the floor. To replenish the used water, you need to move water from the roof tank. And to avoid damages caused by not-so-clean water, you need to purify said water to make it usable. That "purification" process is your DC-DC converter. The used water from the faucet or fountain then get collected and pumped back into the roof reservoir (that process is your MGU) and the cycle repeats.
Now, when your "purificator" is clogged or not working, the small tank is not getting replenished. As such, your "purified" water level decreases as you use it, until you don't have usable water anymore.
This is where you are right now. It could be the "purification device" is clogged/ broken, or your big water tank is broken and leaking.
