I assume it’s largely a side effect from the final RL in post training?

That’s the step that causes the most significant gains in agentic performance.

But the RL doesn’t care about anything except maximizing the score, so if you only score based on coding benchmarks, anything can happen to the writing style (as long as it doesn’t hurt the coding performance).

That’s why it often gets worse on models that simply had more RL post training from the same base.