As you said, the creeping symptom is chiefly a result of the surface finish of the seat. If the end of the piston has taken on anything more than a slight dimple, or has tiny debris pinched into it, or has faint scratches or machining marks visible with a magnifying glass or loupe, it needs to be dressed clean and flat and smooth.
Using good lighting and magnification, also check the orifice to which the piston meets. Granted it's rare to find a problem there but you'll be chasing your tail if there is.
Bear in mind all regulators will eventually develop creep as the seat wears from being cycled many thousands of times. Additionally, there is a compromise between early performance and long term durability that designers have to weigh. For example a somewhat soft seat material like nylon will iron in quickly to the orifice and work very nicely right away, which creates a positive first impression. However it will also wear out and need servicing more quickly than a hard material.
Conversely a hard material like PEEK may creep for a while when new, which of course creates a negative first impression. But then once it has broken in, works great and lasts a long time.
To get the best of both worlds, one can use a hard plastic and spend the time to bring both surfaces up to a very high quality finish. That way it won't creep in the early going and it will last a long, long time. So why don't we see more manufacturer's doing this? Because achieving this level of finish consistently in a production setting is time consuming and costly and prone to QC issues.
The FX piston is Delrin (or at least is commonly thought to be) which a material with properties in between the two examples I listed above. Other things being equal, it is a good material for a regulator's valve seat. However it may not be so good for making the entire piston because it is it has a linear coefficient of thermal expansion over 5x greater than the regulator body, which means the output pressure will change with temperature. And perhaps some other subtle effects arising out of using a slightly compressible/flexible material in an assembly whose whole purpose in life is to go from passing air molecules -> blocking air molecules when a particular pressure is reached.
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