Probably the butane that is not evaporated in the loop. The high pressure in the evaporator is not forcing a low enough boiling point on the butane. I have a sneaky suspicion that you have a lot of liquid butane living in the accumulator after the unit has run up to its balance point. It can only exist at these high pressures as a subcooled liquid, except the tiny fraction that dissolves in the propane due to partial pressure.
Propane Butane Mix - Evaporation Pressure