Disclaimer: I'm no expert
From what research I've done, it seems that the cause of creep is due to poor design flow to the wastegate port. The problem may be as simple as a small casting abnormality which directs flow away from the WG.
That being said, I suspect that altitude will decrease final boost pressure due to lack of atmospheric pressure but the change would not be significant enough to stop creep.
The design from Nameless is a good start by reducing the back pressure at the WG to avoid creep. I'm afraid the casting of your stock turbo may be to blame.
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