Optimizing for contradicting objectives is always hard, this behavior will always leak this way or another. Even basic instruction training will affect writing produced piece by piece, because the structure of a typical assistant reply will leak into each piece, for example making characters use narrative echo to annoying degrees. Older models (that did indeed write better) still had this particular quirk. This can be alleviated by using assistant prefill, and its removal from frontier models is a disaster.

Of course if you optimize for natural-sounding language it will learn to sound more natural.