I expect from the free and ...

I expect from the free and total amounts that the water leaving the works is chloramminated by adding ammonia to convert to monochloramine.  I assume that the free chlorine level means that the breakpoint has been exceeded.

It appears to me that the chlorammination  has been suspended hence the residual now being all as free chlorine. I'm not sure if you are saying that the quality is different at each end of the supply line, with a lower free chlorine at the works which would be hard to  achieve. If this is the case you would need to examine trends to make sure that you are not looking at a transient.

When dropping flows substantially the dosing equipment may not be able to turn down as readily and may  lead to overdosing. Even if it can the process time will change and may throw out control function.