Constant current source drop
Your load requires 15V to maintain 10mA (10mA x 1.5k).
Below about 16V there is no voltage available across Q2 for it to control the current so the current has to drop.
As @mkeith points out the circuit cannot boost the input voltage.
With all constant current circuits, there is a parameter usually called the &quot;Compliance Voltage&quot; which is the range of voltages across the load for which the current is constant.
In this case, the compliance voltage is about 1V less than the input voltage.
With a lower value load resistance the current would be constant down to a lower voltage.