diff --git a/bisect/39931.py b/bisect/39931.py
new file mode 100644
index 0000000000000..7ee49c1ac8963
--- /dev/null
+++ b/bisect/39931.py
@@ -0,0 +1,21 @@
+# BUG: Index alignment behaviour #39931
+
+import numpy as np
+
+import pandas as pd
+import pandas.testing as tm
+
+print(pd.__version__)
+
+np.random.seed(1)
+df = pd.DataFrame(np.random.randint(0, 100, (10, 2)), columns=["A", "B"])
+
+mask = df["A"] >= 40
+
+df2 = df.copy()
+
+df2[mask] = df[mask].sort_values("A")
+print(df2[mask])
+
+
+tm.assert_frame_equal(df, df2)