question

The only time the smart shunt will be accurate is when it synchronizes at absorption voltage and tail current, i.e., 100% SoC. It does not factor voltage into % charge at all.

A smart shunt power loss can force a 100% reading depending on settings.

If you're only getting your battery to 13.4V, then you're nowhere near fully charged, and you haven't synchronized in a long time, thus your smartshunt accuracy is way off. Typical AGM are fully charged at 14.2-14.8V depending on manufacturer.

If this is normal operation for you, then you have been damaging your batteries every single day you failed to fully charge them. You likely have way too little solar for your daily consumption and are using your AGM batteries in the lower to mid range of their state of charge, which damages them.

Recommend you immediately take all necessary measures to fully charge your battery in accordance with the manufacturer's specifications.

Error:

https://www.victronenergy.com/media/pg/CCGX/en/vrm-portal.html#:~:text=Error%20%23153%20Connection%20problem%2C%20and,and%20also%20the%20time%20zone.

An SOC sync can falsely occur if the battery voltage is above the "charged" threshold and the charging current drops below the tail threshold. This can happen when clouds block the sun. Just a thought.