I’m not sure I would call it a “bug”, per se. It still works “as intended”.
Obsidian forces CSS upon MermaidJS initalisation, as kind of a “catch-all, theme agnostic” kind of setup.
It works nice for most older diagrams, but Obsidian uses a version where sankey is in beta (technically, sankey diagrams are still in beta/experimental as of MermaidJS 10.9.1).
However, it breaks for diagrams like sankey, simply because of how the diagrams are structured in the SVG compared to the other diagrams.
So, not a “bug”, per se, but also not entirely feature-friendly; it’s implemented in a quick and dirty roundabout way, making the other (older) diagram types look like they belong in any theme without having to add lots of stuff into the stylesheet.
With that said, I doubt the Obsidian team will do anything about this for as long as sankey is still “experimental”.