Update: Under Qt 5.5.0 release, this bug has reappeared. Added UdpTest2.zip.

Under Qt 5.3.0-Alpha, my QUdpSocket implementation no longer receives readyRead signal. I can still see packets via Wireshark, but no readyRead is received.

Did a diff on qabstractsocket.cpp 5.2.1/5.3.0, which revealed a change at lines 751/752.
5.2.1:
if (!hasData && socketEngine)
socketEngine->setReadNotificationEnabled(true);
5.3.0-Alpha:
if (isBuffered && socketEngine)
socketEngine->setReadNotificationEnabled(readBufferMaxSize == 0 || readBufferMaxSize > q->bytesAvailable());

Changing code back from 5.3.0-Alpha to 5.2.1 fixes it.