Maybe the voltage range that you used is not suitable, and thus the PBA also deposited on the reference electrode.

Dear Bin

I checked the voltage, I assume that Prussian blue is formed in the solution of electrolyte and deposit on the Ref electrode physically rather than electrochemically

Hello,

Its a common phenomenon that while using any strong dyes, dye molecules stack onto the surface of reference electrode by physisorption. So I think using a certain reference electrode for working with dye molecules is good to keep other reference electrodes uncontaminated.