Why does Sulfur dioxide incompletely freeze when entrained in a helium stream inside tubing dipped in liquid nitrogen?
Phase diagrams are about thermodynamics, while you have to consider kinetics as well.
The stream may have too high linear speed at 30+ mL/min to reach equilibrium while still in the loop.
Another  option is dragging  particles of/containing SO2 by the gas stream by mechanical way, not allowing it to settle. Like wind taking snowflakes, not allowing them to land and stay.