Why can SOC show 100% yet batteries won't charge beyond 13.5 volts?

Are now in winter, you solar may not be generating enough power to get them right up to 14.2V. Remember, if the batteries are at 13.4V to 13.5V they are pretty well full. 13.4V with a small current draw is going to be over 90% full.

One thing is your BMV “Charged Voltage” may not be set ideally for solar in winter and going to 100% too quickly. Have a look at the link below

BMV reading 100% when charging at lower voltage