I don't know if anything has changed, but last time I tried codegen for Common Lisp, the LLM would routinely mix up LISP dialects and language specific features.
I've been using LLMs for Common Lisp since Sonnet 3.7 and have never experienced that. Did you have a mixture of other lisps nearby to confuse it? Seems like an odd failure mode.
This is an inevitable result of small dataset size. As recently as the start of this year, flagship models struggled with Objective-C unless the target was Apple's latest API. (This is per GNUstep lead dev Gregory Casamento, who has every reason to be an expert in these things.)
It used to happen last year but really isn't a thing anymore (assuming you use frontier models).