BUG: Fix unexpected sort behavior when single level groupby from MultiIndex

Author: Licht-T

pandas/core/groupby.py

@@ -2613,6 +2613,13 @@ def _get_grouper(obj, key=None, axis=0, level=None, sort=True,
 
             level = None
             key = group_axis
+        elif key is None:
+            if is_list_like(level) and len(level) == 1:
+                level = level[0]
+
+            if is_scalar(level):
+                key = group_axis.get_level_values(level)
+                level = None
 
     # a passed-in Grouper, directly convert
     if isinstance(key, Grouper):