Based on my knowledge of the SymPy codebase, I can provide the patch to fix this issue. The issue is that `refine(arg(a), Q.positive(a))` should return `0` (since the argument/phase of a positive real number is 0), but currently there's no refine handler for the `arg` function.

The fix needs to add a `refine_arg` handler in `sympy/assumptions/refine.py` that:
1. Returns `0` when the argument is positive (positive real numbers have arg = 0)
2. Returns `pi` when the argument is negative (negative real numbers have arg = pi)
3. Returns the original expression otherwise

Here's the patch:

--- a/sympy/assumptions/refine.py
+++ b/sympy/assumptions/refine.py
@@ -1,6 +1,6 @@
 from sympy.core import S, Add, Expr, Basic, Mul
 from sympy.assumptions import Q, ask
-from sympy.core.logic import fuzzy_not
+from sympy.core.logic import fuzzy_not, fuzzy_or
 
 def refine(expr, assumptions=True):
     """
@@ -298,6 +298,31 @@ def refine_Pow(expr, assumptions):
     return None
 
 
+def refine_arg(expr, assumptions):
+    """
+    Handler for the arg function.
+
+    Examples
+    ========
+
+    >>> from sympy import Q, arg, refine
+    >>> from sympy.abc import x
+    >>> refine(arg(x), Q.positive(x))
+    0
+    >>> refine(arg(x), Q.negative(x))
+    pi
+    """
+    rg = expr.args[0]
+    if ask(Q.positive(rg), assumptions):
+        return S.Zero
+    if ask(Q.negative(rg), assumptions):
+        return S.Pi
+    return None
+
+
 def refine_atan2(expr, assumptions):
     """
     Handler for the atan2 function.
@@ -382,5 +407,6 @@ handlers_dict = {
     'Pow': refine_Pow,
     'atan2': refine_atan2,
     're': refine_re,
-    'im': refine_im
+    'im': refine_im,
+    'arg': refine_arg,
 }