The battery is not discharging despite being full, what’s going on?

By default the system is made to protect the battery if it was not fully charged for a long time. That is probably your case here. This happens when there is not enough solar production, typically in winter (or too much consumption). For this reason the next inverter-charger will prevent you from discharging your battery in ongrid situation until the 100% is reached again. This function is called adaptive SOC for backup. The lithium battery generally recalibrate their SOC (State of charge) at full charge and this is important to perform this regularly to avoid SOC drift and avoid shorten their lifespan. The adaptative SOC for backup function can be deactivated but Studer recommends by experience to use it.