You are a person, LLMs are not. You know this, which is why you know that if you signed someone else's name it would be forgery, but when you see the machine do it you call it a bug.

If the machine is like you, the machine is a forger. The machine is not like you, it is simply blending the work of others to order. Adding someone else's signature is simply part of that statistical process.

I think this is a tangent. The obvious problem with signing someone else's name to work they did not create and do not endorse is that, if you publish the image, you will be falsely attributing authorship to them without their consent. The intent (if any) behind the addition of the signature is a separate issue.