diff --git a/src/corelib/serialization/qcborvalue.cpp b/src/corelib/serialization/qcborvalue.cpp
index 63ee5f3158..ebb3665e0c 100644
--- a/src/corelib/serialization/qcborvalue.cpp
+++ b/src/corelib/serialization/qcborvalue.cpp
@@ -994,8 +994,12 @@ void QCborContainerPrivate::replaceAt_complex(Element &e, const QCborValue &valu
         e = value.container->elements.at(value.n);
 
         // Copy string data, if any
-        if (const ByteData *b = value.container->byteData(value.n))
-            e.value = addByteData(b->byte(), b->len);
+        if (const ByteData *b = value.container->byteData(value.n)) {
+            if (this == value.container)
+                e.value = addByteData(b->toByteArray(), b->len);
+            else
+                e.value = addByteData(b->byte(), b->len);
+        }
 
         if (disp == MoveContainer)
             value.container->deref();
@@ -2649,7 +2653,7 @@ void QCborValueRef::assign(QCborValueRef that, QCborValue &&other)
 void QCborValueRef::assign(QCborValueRef that, const QCborValueRef other)
 {
     // ### optimize?
-    assign(that, other.concrete());
+    that = other.concrete();
 }
 
 QCborValue QCborValueRef::concrete(QCborValueRef self) noexcept
diff --git a/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp b/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
index 6d8161c1f9..c70518fbee 100644
--- a/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
+++ b/tests/auto/corelib/serialization/qcborvalue/tst_qcborvalue.cpp
@@ -79,6 +79,7 @@ private slots:
     void mapEmptyDetach();
     void mapSimpleInitializerList();
     void mapMutation();
+    void mapMutateWithCopies();
     void mapStringValues();
     void mapStringKeys();
     void mapInsertRemove_data() { basics_data(); }
@@ -923,6 +924,76 @@ void tst_QCborValue::mapMutation()
     QCOMPARE(val[any][3].toMap().size(), 1);
 }
 
+void tst_QCborValue::mapMutateWithCopies()
+{
+    {
+        QCborMap map;
+        map[QLatin1String("prop1")] = "TEST";
+        QCOMPARE(map.size(), 1);
+        QCOMPARE(map.value("prop1"), "TEST");
+
+        map[QLatin1String("prop2")] = map.value("prop1");
+        QCOMPARE(map.size(), 2);
+        QCOMPARE(map.value("prop1"), "TEST");
+        QCOMPARE(map.value("prop2"), "TEST");
+    }
+    {
+        // see QTBUG-83366
+        QCborMap map;
+        map[QLatin1String("value")] = "TEST";
+        QCOMPARE(map.size(), 1);
+        QCOMPARE(map.value("value"), "TEST");
+
+        QCborValue v = map.value("value");
+        map[QLatin1String("prop2")] = v;
+        QCOMPARE(map.size(), 2);
+        QCOMPARE(map.value("value"), "TEST");
+        QCOMPARE(map.value("prop2"), "TEST");
+    }
+    {
+        QCborMap map;
+        map[QLatin1String("value")] = "TEST";
+        QCOMPARE(map.size(), 1);
+        QCOMPARE(map.value("value"), "TEST");
+
+        // same as previous, but this is a QJsonValueRef
+        QCborValueRef rv = map[QLatin1String("prop2")];
+        rv = map[QLatin1String("value")];
+        QCOMPARE(map.size(), 2);
+        QCOMPARE(map.value("value"), "TEST");
+        QCOMPARE(map.value("prop2"), "TEST");
+    }
+    {
+        QCborMap map;
+        map[QLatin1String("value")] = "TEST";
+        QCOMPARE(map.size(), 1);
+        QCOMPARE(map.value("value"), "TEST");
+
+        // same as previous, but now we call the operator[] that reallocates
+        // after we create the source QCborValueRef
+        QCborValueRef rv = map[QLatin1String("value")];
+        map[QLatin1String("prop2")] = rv;
+        QCOMPARE(map.size(), 2);
+        QCOMPARE(map.value("value"), "TEST");
+        QCOMPARE(map.value("prop2"), "TEST");
+    }
+    {
+        QCborMap map;
+        map[QLatin1String("value")] = "TEST";
+        QCOMPARE(map.size(), 1);
+        QCOMPARE(map.value("value"), "TEST");
+
+        QCborValueRef v = map[QLatin1String("value")];
+        QCborMap map2 = map;
+        map.insert(QLatin1String("prop2"), v);
+        QCOMPARE(map.size(), 2);
+        QCOMPARE(map.value("value"), "TEST");
+        QCOMPARE(map.value("prop2"), "TEST");
+        QCOMPARE(map2.size(), 1);
+        QCOMPARE(map2.value("value"), "TEST");
+    }
+}
+
 void tst_QCborValue::arrayPrepend()
 {
     QCborArray a;