I don't know if anyone here has any experience of the CBOR node, so I doubt you'll get a direct answer.
But we can help you try to figure it out.
I assume you mean
node-red-contrib-cbor - it is barely complete node, with no help text.
Looking at its code, I can see it will only decode
msg.payload if it's a Buffer type object. So the question what is the MQTT node passing it. You can configure the MQTT node to always send a Buffer type object - I suggest you do that it you haven't already. It's always worth adding a Debug node so you can check exactly what you are passing into the CBOR node.