Thank you for pointing me to that, it helped a lot! It’s still a little weird, but at least I have a workaround now.

-> sent
{
  id: '16',
  op: 'eval',
  session: '8df8145d-ecfe-41e1-9e99-9e9ce33861aa',
  code: '^{:portal.viewer/default :portal.viewer/hiccup}[:h1 "hello, world"]',
  file: '/Users/lukas/Workspace/clojure/calva/test-data/notebook.clj',
  line: 37,
  column: 3,
  stdout: [Function: stdout],
  stderr: [Function: stderr],
  pprint: false,
  'nrepl.middleware.print/print': 'nrepl.util.print/pr',
  'nrepl.middleware.print/options': { 'print-meta': true }
}

<- received
{
  id: '16',
  ns: 'notebook',
  session: '8df8145d-ecfe-41e1-9e99-9e9ce33861aa',
  value: '^#:portal.viewer{:default :portal.viewer/hiccup} [:h1 "hello, world"]'
}

You're welcome!