When you use storage mode the charger is pretty restricted on how much power it can dump if you start out above the storage target level, it's probably only a handful of amps. Less than it can push in while charging. And probably less than 10% of what you pull out while racing. Easier to just blast round in the car for 10 minutes first. Then do it.
You'll probably find that 8.4v is a bit inaccurate, or the cells are slightly out of balance, so it's trying to get them both to 4.2, but using the tiny tiny little wires and a trickle charge, so it takes ages.
When you restart it might just think "ah, near enough" and leave them as is.
Might be worth seeing if your charger can be calibrated to make sure that 4.2v/cell is actually accurate.
Also worth checking your charge cables are good, cleanly soldered etc. Bad cables can add a small amount of resistance to the circuit, confusing the charger.
Not that i've ever had badly solder cables. Oh No. Not me.
