I'm having the same issue with the mermaid servers since a week ago (or so). I tried running the server locally on docker and it works as expected. For the issue with node names I'm assuming your Jupyter notebook is caching the image for a previously created graph and when you use that old name it just uses the cache hence you don't see an error. Try creating a new notebook (or even better a whole new python environment), and run both codes in there see if you can reproduce the error for both versions.

You can try and run mermaid server locally following these steps. Let me know after you try these, we need to report this to langchain and mermaid team.

1. Download and run the mermaid container folowing instrutions here.
2. From your python directory navigate to langchain_core/runnables/graph_mermaid.py/ > _render_mermaid_using_api() and replace https://mermaid.ink in image_url with `http://localhost:3000. Keep the rest of the url intact.