Flatbuffer object returning null on Netty Bytebuffer

We are using Flatbuffer with Java bindings. The application is running Netty4 and has following code for handling Netty request:

String id;
try {
        request = PostRequest.getRootAsPostRequest(msg.getPayloadBuffer().nioBuffer());
        id = request.id(); // string

    } catch (final Exception exception) {
        ctx.writeResponseAndFlush(StatusCode.StatusCode400, exception);
        return;
    } finally {
        msg.getPayloadBuffer().release();
    }

traceId = request.traceId(); // <-- this string sometimes becomes null

This code occasionally sets traceId as null and I am not able to fully understand why. Is it because the underlying buffer might have been release and reused as I am releasing it in finally block?

Even then, why does it happen only rarely and not always?