(SFFR) is a service deployed by National Grid ESO, also intended to support the grid system during frequency disturbances. Although this one is only in one direction - providing energy when there is a shortfall.
When you system provides SFFR, your battery is likely to maintain a certain level of energy on the battery until the event happens, this can be for a few weeks. But you as the provider of the service you are paid to store that energy until such time that it is needed (as the time and date is not predictable).
When you battery isn't discharging it can be concerning, but this is perfectly normal, and the credits you receive will more than cover the cost of the energy you had to import due to providing this service.