question

@kim8skeptic has it.

To answer your original question, since the BMV and the battery have been out of sync, it's possible you've deeply discharged the batteries thinking you were above 60% while they were actually much lower, especially since you're in an area without a lot of daylight hours currently.

On the graphs you provided, I do not see indications of a false sync. Those are indicated by an abrupt jump to 100%, so you may really be dealing with degraded capacity.

Lastly, 2 years out of cheap Chinese SLA may be all they're good for, and they may never have met ratings.

Recommend the following changes:

BMV:

The idea behind setting SoC to 50% is to ensure that the next time you see 100%, it's because it's actually met the full charged criteria.

MPPT:

You should really consider generator charging if it's an option.

Other concern:

While not outrageous, your midpoint variation is of concern. Bottom voltage is clearly always a little higher than top voltage. I assume they're wired in a standard 2S2P configuration, and you're just monitoring bottom voltage on one string of 24V. Recommend you measure all 4 12V near the end of their absorption cycle and shuffle them in the bank. Match the two highest voltages in a 24V string and the two lowest voltages in a 24V string and then parallel the two strings at the ends. Also, make sure you're main leads are connected across the bank, i.e., (+) is on one string and (-) is on the other string.

Lastly, you may want to correlate resting voltage with SoC as follows:

Take the system completely offline first thing in the morning before charging begins. Allow the battery to sit at rest (no charge or discharge) for 2 hours. Measure system voltage and 12V votlages. Compare to this chart:

Hopefully you'll see them matching within 10%.