The darlington and the common emitter transistor driving it is just a very high gain current multiplier. Its not fixed bias, but bias is controlled by the current through the transistor.
The output current into the load is monitored by a current shunt, and that provides feedback back into the proportional integral error amplifier.
The control input at the upper left sets the desired current through the load, and the feedback amplifier ensures that those conditions are met.
The error amplifier need to be correctly tuned for both gain and phase, to obtain best transient response and stability.
If its not tuned correctly, the circuit may either have a very sluggish response to rapid input changes, or it may overshoot, and become unstable, even oscillate.
If you feed an appropriate frequency square wave drive signal into the input, and monitor the load current with an oscilloscope, that will tell you all you need to know about how well the feedback amplifier is working, or what may be wrong with it.