I've tracked it down to the fact that when QNativeSocketEnginePrivate::nativeWrite() calls WSASend() it returns SOCKET_ERROR and the reason is WSAEWOULDBLOCK, now this appears to be because under normal HTTP, only chunks of 4096 are sent which are fine but using SSL, chunks of 30kB are sent which causes the socket to still be busy on the next attempt. This is find expect that somewhere it seems to report that the bytes were sent and read more from the input device.
That is as far as I've got with it.
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