fixed overriding user provided strings and added debug message

Author: digicosmos86

pymc/sampling/parallel.py

@@ -386,9 +386,13 @@ def __init__(
         if mp_ctx is None or isinstance(mp_ctx, str):
             # Closes issue https://github.com/pymc-devs/pymc/issues/3849
             # Related issue https://github.com/pymc-devs/pymc/issues/5339
-            if platform.system() == "Darwin":
+            if mp_ctx is None and platform.system() == "Darwin":
                 if platform.processor() == "arm":
                     mp_ctx = "fork"
+                    logger.debug(
+                        "mp_ctx is set to 'fork' for MacOS with ARM architecture. "
+                        + "This might cause unexpected behavior with JAX, which is inherently multithreaded."
+                    )
                 else:
                     mp_ctx = "forkserver"