SFFR is a service deployed by National Grid ESO which supports the grid system during frequency disturbances. Although this one is only in one direction - providing energy when there is a shortfall.
When your 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 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 your battery isn't discharging it can be concerning, but this is perfectly normal, and the credits you receive will cover the costs related to providing this service.