problem to clamp a signal with adp1720

Hello,

     I'm using an ADP1720 - Adj as a 2.5V power supply.

     By using a resistor (2.21K) and a diode (BAV99) a differential amplifier (AD8221) output is clamped to 2.5V .

     When the amplifier output is 12V, this signal is clamped to 4V instead of 2.5V and the power supply value change from 2.5V to 4V.

     Is it normal ???