Back pressure valves and control valves are used in the liquid addition pipeline

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In the liquid addition pipeline, a control valve is used to regulate the flow rate. To maintain a stable pressure difference across the control valve, a backpressure valve is installed at its outlet. When the control valve is open at 100%, the back pressure is adjusted to 3.5 barg. When the opening degree of the control valve was reduced from 80% to 30%, it was observed that the pressure at the backpressure valve dropped to 3.0 barg, while the liquid continued to flow out. If a back pressure of 3.5 barg is set, then when the pressure in the pipeline falls below 3.5 barg, the back pressure valve should close, creating a pressure buildup; flow will only resume once the pressure rises to 3.5 barg. When the opening degree of the control valve decreases, pressure loss will inevitably occur at the outlet. At this point, the back pressure valve should come into action to create a pressure hold, ensuring that the pressure at the outlet remains at 3.5 barg at all times. What could be the reasons for this drop in pressure, along with the leakage of liquid? Is there a problem with the diaphragm or spring of the backpressure valve? Can’t be fully compressed? Please ask the experts to help analyze it