fix: Use different payloads for different chains

stan/model.py

@@ -169,7 +169,7 @@ def _create_fit(self, payload: dict) -> stan.fit.Fit:
                 "save_warmup",
                 arguments.lookup_default(arguments.Method["SAMPLE"], "save_warmup"),
             )
-            payloads.append(payload)
+            payloads.append(payload.copy())
 
         async def go():
             io = ConsoleIO()