In cascode connection, the lower device provides voltage gain, the upper device isolation between drain and the lower gate.
If the upper gate is fed from a fixed dc voltage source (as it usually is) this pretty much fixes the current through the whole device, and sweeping the drain voltage will have little or no effect on drain current once drain voltage is above some minimal threshold.
That is the whole idea behind the cascode connection.
You can get extremely high voltage gain without the capacitive "Miller"coupling back between the drain (output) and the lower gate (input).
The whole device will act pretty much as a constant current sink independent of drain voltage over a very wide range.