This is an under warranty service event or an exchange with the retailer. I would "take it back to the store" for exchange myself. I realize you said "just received" which may imply it was mailed to you. Even under these conditions, returns should be manageable with the retail shop. My past instruments - even before covid hit - was through an out-of-state shop. Things have not always arrived in perfect shape and they have always made it right. Not to say you have the same circumstances - only that this is the route I would prefer to take since it makes me feel better I get a second shot at perfection rather than patching up a device I know had one issue coming out of the gate.

If you'd prefer taking the service route - you would use the service locator link associated with your geographic region.

https://www.yamaha.com/paragon/servicerlocator/
Then click on "Synthesizers & Music Production", then "Synthesizers"

... would be associated with my region (US).

Hello,

I have just received my Reface CS and one of the keys (A3) is very faulty. When using the synth and its internal speakers, it only picks up on 50% of the times I press it. When used in Midi mode, plugged in via USB to logic, the A3 key mostly only triggers a maximum velocity signal, and sometimes nothing.

Any advice greatly appreciated.
Many thanks.

You should contact the dealer where you purchased it and get it replaced. We cannot be sure if it is under warranty or not, or even if you purchased it from an authorized dealer. But either way, an authorized dealer will likely just replace it. If you purchased from an individual or just found it on eBay (whatever) you should immediately contact the seller.
We can do nothing here but give you advice and tell you that what you are reporting is not what a working refaceCS does.

Let us know.