Yes that one battery at 5.95v is most likely dying. When it drops below 6v it pulls power from the other batteries to try and get above 6v causing the lift to have issues. Luckily you've caught it early. Currently the drop is around 10%. If you wait for the battery to drop lower then it will make this issue more visible. With it at 100% and the one battery at 5.95v that is a huge red flag. I recommend replacing that battery asap before it causes damage to th3 other 3 batteries.

Code 218 just means that the throttle pot is seeing voltages that are out of the calibrated range. That pot could be going bad, but there are alot of other checks that can be done to narrow the issue down further.