diff --git a/src/corelib/serialization/qxmlstream.cpp b/src/corelib/serialization/qxmlstream.cpp
index 50f87900161..182f3d7e1e8 100644
--- a/src/corelib/serialization/qxmlstream.cpp
+++ b/src/corelib/serialization/qxmlstream.cpp
@@ -572,6 +572,7 @@ void QXmlStreamReader::addData(QAnyStringView data)
         } else if constexpr (std::is_same_v<decltype(data), QLatin1StringView>) {
             // Conversion to a QString is required, to avoid breaking
             // pre-existing (before porting to QAnyStringView) behavior.
+            d->lockEncoding = true;
             if (!d->decoder.isValid())
                 d->decoder = QStringDecoder(QStringDecoder::Utf8);
             addDataImpl(QString::fromLatin1(data).toUtf8());
diff --git a/tests/auto/corelib/serialization/qxmlstream/tst_qxmlstream.cpp b/tests/auto/corelib/serialization/qxmlstream/tst_qxmlstream.cpp
index d40cb8fa8d0..d0821781222 100644
--- a/tests/auto/corelib/serialization/qxmlstream/tst_qxmlstream.cpp
+++ b/tests/auto/corelib/serialization/qxmlstream/tst_qxmlstream.cpp
@@ -568,6 +568,7 @@ private slots:
     void readFromQBuffer() const;
     void readFromQBufferInvalid() const;
     void readFromLatin1String() const;
+    void readLatin1Document() const;
     void readNextStartElement() const;
     void readElementText() const;
     void readElementText_data() const;
@@ -1229,6 +1230,25 @@ void tst_QXmlStream::readFromLatin1String() const
     }
 }
 
+void tst_QXmlStream::readLatin1Document() const
+{
+    const auto in = "<?xml version=\"1.0\" encoding=\"iso-8859-1\"?><a>M\xE5rten</a>"_L1;
+    {
+        QXmlStreamReader reader(in);
+        QVERIFY(reader.readNextStartElement());
+        QString text = reader.readElementText();
+        QCOMPARE(text, "M\xE5rten"_L1);
+    }
+    // Same as above, but with addData(), QTBUG-135033
+    {
+        QXmlStreamReader reader;
+        reader.addData(in);
+        QVERIFY(reader.readNextStartElement());
+        QString text = reader.readElementText();
+        QCOMPARE(text, "M\xE5rten"_L1);
+    }
+}
+
 void tst_QXmlStream::readNextStartElement() const
 {
     QLatin1String in("<?xml version=\"1.0\"?><A><!-- blah --><B><C/></B><B attr=\"value\"/>text</A>");