Strange common signal due to the protection resistor of the bias electrode

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I’m designing an EEG system and actually I’m managing the bias feedback to remove the common signals. Due to the standard imposed from the international medical associations, for safety it takes a protection resistor to prevent significant current flowing through the user body.
Initially I placed the resistor inside the feedback of the filter which is a part of my bias driver. It seems to work but a 50Hz component remain whatever the filter gain is.

But if I move the protection resistor from the inside to the outside of the feedback filter the component disappear.

Did you ever seen this behavior? Can anyone explain the reason of it?