Lifeline's tech manual pdf does state that in lieu of being able to meet the minimum 20% rate when deeply cycled, that one can apply a constant current of 2 amps per 100Ah of battery for no more than an hour.
page 20They do not mention what this 2 amps of constant current might push voltage upto, but I suspect well over 16v.
I've not read in detail or too closely how your system is monitored, but I need to reset my Amp hour counter on a regular basis, and the more high amp recharging I do the more often this needs to be reset, at least with my battery monitor( Blue sky IPN pro remote)
I reset it when I know the battery is full, and I determine that by amps it accepts at absorption voltage, and it is best if I high amp recharge it to that absorption voltage and hold it until amps taper to that level.
Usually when i notice it is out of whack with reality it reads a 6 to 8Ah from full when amps taper to 0.45, but there have been times when it said 0Ah from full when it was still accepting 4 amps and the tapering takes another 2 hours.
No great when one has to monitor the monitor, but so be it.
I am of the opinion that when my AGM is appearing to not hold as high a voltage as it should during discharge, that it is time for a high amp recharge, and the longer I can apply those high amps, the more I heat the battery, the better it seems to perform thereafter until it accumulates more partial state of charge cycles or low and slow solar only recharges.
So when my battery is showing signs of petulance, I drain it well below 50%, in 6 or 7 hours, then combine my 40 amp meanwell power supply with my 25 amp schumacher charger, for 65 amps into a battery rated at 90Ah when new.
It does gain a bunch of temperature doing this, but takes 65 amps for 22 minutes the last time I did this, before it reached 14.7v, and gained at least 15f of temperature, iirc, and temp kept rising until amps tapered below 4.
So I'd drain 2 of your 4 lifelines lower than 75% SOC, quickly and hammer them with the magnum's full output until amps taper, then do the same to the other pair, and note how long each pair takes to reach absorption voltage and how much longer it takes for amps to taper to 0.5% of capacity.
You might just have one underperforming battery of the 4.
If your AC can power a secondary charger, many of us parallel chargers for higher amperages/ faster bulk charging.
Might also want an IR temp gun and see if any connections are getting abnormally hot or hotter than the others during these higher amperage episodes.
That includes the Shunt.