I also ran into massive issues trying to use Mermaid when I tried to pair it up with Marp (markdown for presentations). I am not sure if it is Mermaid's fault or Marp's fault but it didn't work and tons of people want it to work:

https://github.com/orgs/marp-team/discussions/207

The discussion seems to make it pretty clear where the issues are, though, and how you can get around them using mermaid's render to svg.