I’m well aware. It’s also a one line change to the myst parser to make that work.
In any case, it’s because a myst code block with curly brackets is really a sphinx directive.
That said, they should support the non-directive version, similar to GFM, but would
need to embed it. Compared to GFM, though, a plug-in can be added to support plantUML and there’s even crusty ones for graphviz.
GFM version is also not particularly flexible for large diagrams
If you want an example of this breakage, try embedding a Mermaid format diagram in both of them.