If a belt breaks their responsibility ends with the contractor, it relies on the baggage handlers to move to another belt or manually transfer luggage if needed, and yes apron services have action plans in place in the event of failure, additionally they have between 12am and 4-5 am to take care of any maintance requirements.
In other words while providing the infrastructure, and the subcontractor they have zero liability and responsibility other then being the enforcer. In other words if baggage goes missing, or is damaged it is up to the airline and apron services to mediate. While the airline will take immediate responsibility in some instances, they too shift responsibility to that of apron services or any other baggage handler.
So ACSA's physical responsibility stops at providing the infrastructure, this will also mean in some instances belt equipment belonging to the actual baggage handler or sub contractor.