--- a/mlflow/server/__init__.py
+++ b/mlflow/server/__init__.py
@@ -474,6 +474,11 @@
             else tempfile.mkdtemp()
         )
+    # In Nix-packaged environments sys.path is populated by wrappers but
+    # PYTHONPATH is never set in os.environ, so subprocesses (uvicorn,
+    # gunicorn) cannot find packages. Propagate it when not already set.
+    if "PYTHONPATH" not in os.environ:
+        env_map.setdefault("PYTHONPATH", os.pathsep.join(p for p in sys.path if p))

     server_proc = _exec_cmd(
         full_command,
         extra_env=env_map,