Not everything is about being 100% efficient.

LLM didn't discover this issue, developers found it. Instead of fixing it themselves, they intentionally turned the problem into an issue, left it open for a new human contributor to pick up, and tagged it as such.

If everything was about efficiency, the issue wouldn't have been open to begin with, as writing it (https://github.com/matplotlib/matplotlib/issues/31130) and fending off LLM attempts at fixing them absolutely took more effort than if they were to fix it themselves (https://github.com/matplotlib/matplotlib/pull/31132/changes).

And then there's the actual discussion in #31130 which came to the conclusion that the performance increase had uncertain gains and wasn't worth it.

In this case, the bot explicitly ignored that by only operating off the initial issue.