Not sure how your SOC meter actually determines/ guesses, but it sounds like it can be one of the issues of your batteries early demise.
I have one 90Ah battery, my battery monitor is set to 80AH. I never ever look at the % screen, only the AH from full.
If it says 7AH from full but the battery is accepting 0.4amps at absorption voltage, I know it is lying
If it says 0AH from full but the battery is still accepting 2 amps at 14.7v, I know it is lying.
If my charge controller decided when to drop to float voltage by the battery monitor's % reading, or AH from full rating, I'd take it out back and shoot it.