I'm not familiar with that specific regulator but I can offer some general thoughts.
What pressure are you filling your reservoir to? Sometimes regulators are more prone to creep with fill pressure is approaching 300 bar. Perhaps keeping below 250 bar might help. When the main seal separating the air reservoir from the plenum takes a set or deforms and begins to leak, the plenum will pressurize higher than the regulator's set pressure. That is the definition of creep that I am familiar with. Lubricating the O-ring seals with silicone grease on all surfaces, inside the groove and not just on the OD of the rings will help with overall sealing ability. This means removing an O-ring completely, coating it with grease, then installing it. If it's not fully lubed at the factory, this could be a weak link.
If a leaking seal is not the problem then is this regulator prime to stiction where it cannot mechanically maintain its setting? If so, then perhaps some polishing and lubrication can help.
I've watched videos from Ernest Rowe where he is tuning up a regulator by polishing all of the bearing surfaces for the seals and the Belleville washers using polishing paste such as flitz and 3M abrasive papers. Then all of the surfaces have been wiped with a sparing amount of silicone grease, just a thin film wiped with fingers.
I don't see any shiny spots on your Belleville washers where the edges touch so they are definitely not broken in yet. I polished mine by applying flitz polishing paste and rubbing them on old bits of terry cloth. You can see smooth shiny areas appear after this. I did this to my Edgun regulators. Here are some pics from the Leshiy Classic regulator tune up I did and you can see the polished shiny circles on the washers for instance. I notated and polished surfaces where O-rings slide when the regulator piston makes its tiny movements after each shot cycle. This should give you some ideas for perhaps tuning up your own reg. Make sure to find out where the tiny valve sealing surface face is and little air passages and make sure everything is free of debris, smooth and clean.
Your regulator parts look pretty smooth and fairly well polished in the photo as far as I could tell. The construction is different so I'm sure others who have done this will have more to offer.
GL
Feinwerk
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