diff --git a/src/serializers/type_serializers/union.rs b/src/serializers/type_serializers/union.rs
index 72cbc34d4..ac674ef34 100644
--- a/src/serializers/type_serializers/union.rs
+++ b/src/serializers/type_serializers/union.rs
@@ -180,7 +180,8 @@ fn serde_serialize<S: serde::ser::Serializer>(
         }
     }
 
-    if retry_with_lax_check {
+    // If extra.check is SerCheck::Strict, we're in a nested union
+    if extra.check != SerCheck::Strict && retry_with_lax_check {
         new_extra.check = SerCheck::Lax;
         for comb_serializer in choices {
             if let Ok(v) = comb_serializer.to_python(value, include, exclude, &new_extra) {
@@ -189,8 +190,14 @@ fn serde_serialize<S: serde::ser::Serializer>(
         }
     }
 
-    for err in &errors {
-        extra.warnings.custom_warning(err.to_string());
+    // If extra.check is SerCheck::None, we're in a top-level union. We should thus raise the warnings
+    if extra.check == SerCheck::None {
+        for err in &errors {
+            extra.warnings.custom_warning(err.to_string());
+        }
+    } else {
+        // NOTE: if this function becomes recursive at some point, an `Err(_)` containing the errors
+        // will have to be returned here
     }
 
     infer_serialize(value, serializer, include, exclude, extra)
diff --git a/tests/serializers/test_union.py b/tests/serializers/test_union.py
index 20ac832a7..66ec3b9b8 100644
--- a/tests/serializers/test_union.py
+++ b/tests/serializers/test_union.py
@@ -958,7 +958,7 @@ def test_union_of_unions_of_models_with_tagged_union_invalid_variant(
     ],
 )
 def test_union_of_unions_of_models_with_tagged_union_json_key_serialization(
-    input: bool | int | float | str, expected: bytes
+    input: dict[bool | int | float | str, str], expected: bytes
 ) -> None:
     s = SchemaSerializer(
         core_schema.dict_schema(
@@ -973,3 +973,30 @@ def test_union_of_unions_of_models_with_tagged_union_json_key_serialization(
     )
 
     assert s.to_json(input, warnings='error') == expected
+
+
+@pytest.mark.parametrize(
+    'input,expected',
+    [
+        ({'key': True}, b'{"key":true}'),
+        ({'key': 1}, b'{"key":1}'),
+        ({'key': 2.3}, b'{"key":2.3}'),
+        ({'key': 'a'}, b'{"key":"a"}'),
+    ],
+)
+def test_union_of_unions_of_models_with_tagged_union_json_serialization(
+    input: dict[str, bool | int | float | str], expected: bytes
+) -> None:
+    s = SchemaSerializer(
+        core_schema.dict_schema(
+            keys_schema=core_schema.str_schema(),
+            values_schema=core_schema.union_schema(
+                [
+                    core_schema.union_schema([core_schema.bool_schema(), core_schema.int_schema()]),
+                    core_schema.union_schema([core_schema.float_schema(), core_schema.str_schema()]),
+                ]
+            ),
+        )
+    )
+
+    assert s.to_json(input, warnings='error') == expected