In the case the output current is pulsed at a maximum the voltage across R12 also is pulsed at a maximum. In this case IC2A hase to deliver up to about 1 Milliampere to get an equilibrium at the inputs. For many opamps this is difficult to attain at very low input voltages. If in this case the output of IC2A has a difference in providing and sinking current, C6 can accumulate charge.
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