Both inline and code block js queries look alright over here using the default theme and no snippets:
# This is not from dataview h1
`$=dv.header(1, "This is from dataview! h1");`
`$=dv.header(2, "This is from dataview! h2");`
~~~dataviewjs
dv.header(2, "This is from dataview! h2");
~~~
Perhaps your theme or some custom CSS is causing the issue?