I have located this to be a mermaid bug that was introduced by upgrading mermaid from v10 to v11. It seems like they decided to do something with lines that are too long (by wrapping them at exactly 200px), but in doing so they are testing the computed width of the element which is sometimes off by a tiny amount from 200px if you have zoomed in or out of the interface (In my testing I keep getting something like 199.98xxxxxx).
I’ve made a patch to fix this that will be included in one of the next updates.