The first stages are designed to essentially turn into an ice block and still function.
This is a Poseidon MK3 after an ANSTI test and may well have been the Norsok test. The ports that aren't frozen are exposed to ambient water and won't freeze since the cooling occurs on the main body.
In the second stages, the key is keeping the ice out of the barrel if it's a "normal" design, and with Poseidon, the gas doesn't really come out near the one moving part, so the odds of it freezing are pretty negligible. The servo valve only bleeds a very small out of gas and the big diaphragm in the middle releases the rest. The ice can't form there for a myriad of reasons.