I have a feeling that your SmartShunt is going to 100% before your batteries are actually full with the charged voltage set to 13.5V. Then you think you have full batteries but you do not. Normally I recommend setting it 0.2V below absorption, so 14.0V. Therefore, it goes to 100% when the voltage is above 14.0V and the current less than 5% of 600 = 30A. Your solar could easily trigger this in poor light.
Change the SmartShunt settings and give your batteries a good charge getting them up to 14.2V, then you know they will be full and the SOC is 100% and then work from there and see if you are getting low voltage. My feeling is you are undercharging and not seeing this.
See my FAQ