I am going to assume you mean 12 degree C, so not so cold to be a worry.
You know with lead acid that they need to be regularly charged to 100% for long life and that not charging them to 100% damages them. Have you regularly been getting to full absorption voltage (about 29V). A 30A MPPT to charge 720Ah of batteries is very small. I wonder if you have been undercharging them and they now have reduced capacity. Supply some of the history screenshots.
You need to check what solar power you are generating compared to your use and how long to recharge the batteries after a heavy use. More solar might be the answer, lithium batteries might be as they do not need to be recharged to 100% regularly like lead acid.
If they have not been fully charged regularly, then the SOC becomes inaccurate. It relies on the batteries getting to 100% regularly to resynchronise and correct the drift in the SOC reading.
Finally, what absorption voltage do you have on the MPPT and what is your “charged voltage” setting on the SmartShunt. Share screen shots of your settings. See the link which describes how the SOC can read high and what the SmartShunt settings should be.