diff --git a/expression/integration_test.go b/expression/integration_test.go
index 3efa93eff5d13..6525120d59eb0 100644
--- a/expression/integration_test.go
+++ b/expression/integration_test.go
@@ -2208,6 +2208,22 @@ func TestAggregationBuiltinJSONObjectAgg(t *testing.T) {
 	result.Check(testkit.Rows(`{"first": "json_objectagg_test"}`))
 	result = tk.MustQuery("select json_objectagg(a, null) from t group by a order by a;")
 	result.Check(testkit.Rows(`{"1": null}`))
+
+	// For issue: https://github.com/pingcap/tidb/issues/39806
+	// Optimization shouldn't rewrite the flag of `castStringAsJson`.
+	tk.MustQuery(`
+	select a from (
+		select JSON_OBJECT('number', number, 'name', name)  'a' from
+		(
+			select 1  as number, 'name-1' as name  union
+			(select 2, 'name-2' ) union
+			(select 3, 'name-3' ) union
+			(select 4, 'name-4' ) union
+			(select 5, 'name-5' ) union
+			(select 6, 'name-2' )
+		) temp1
+	) temp
+	where  a ->> '$.number' = 1`).Check(testkit.Rows(`{"name": "name-1", "number": 1}`))
 }
 
 func TestOtherBuiltin(t *testing.T) {
diff --git a/expression/util.go b/expression/util.go
index 22637d1b8617f..e19ec047e43d0 100644
--- a/expression/util.go
+++ b/expression/util.go
@@ -427,8 +427,10 @@ func ColumnSubstituteImpl(expr Expression, schema *Schema, newExprs []Expression
 				return substituted, hasFail, v
 			}
 			if substituted {
+				flag := v.RetType.GetFlag()
 				e := BuildCastFunction(v.GetCtx(), newArg, v.RetType)
 				e.SetCoercibility(v.Coercibility())
+				e.GetType().SetFlag(flag)
 				return true, false, e
 			}
 			return false, false, v