bug: ser/df.replace only replaces first occurence with NAs

pandas/core/array_algos/replace.py

@@ -93,17 +93,18 @@ def _check_comparison_types(
         )
 
     # GH#32621 use mask to avoid comparing to NAs
-    if isinstance(a, np.ndarray):
+    if isinstance(a, np.ndarray) and mask is not None:
         a = a[mask]
-
-    result = op(a)
-
-    if isinstance(result, np.ndarray) and mask is not None:
-        # The shape of the mask can differ to that of the result
-        # since we may compare only a subset of a's or b's elements
-        tmp = np.zeros(mask.shape, dtype=np.bool_)
-        np.place(tmp, mask, result)
-        result = tmp
+        result = op(a)
+
+        if isinstance(result, np.ndarray):
+            # The shape of the mask can differ to that of the result
+            # since we may compare only a subset of a's or b's elements
+            tmp = np.zeros(mask.shape, dtype=np.bool_)
+            np.place(tmp, mask, result)
+            result = tmp
+    else:
+        result = op(a)
 
     _check_comparison_types(result, a, b)
     return result