For context, see #1605 (comment):

Interesting, the Hello World tabbed pane in the User Guide stopped working June 5 when we upgraded to Hugo 0.112.3 (so it has nothing to do with recent changes):

• Make docsy theme point to latest version of dependencies #1533 - preview https://deploy-preview-1533--docsydocs.netlify.app/docs/adding-content/shortcodes/#tabbed-panes - tabbed pane still works
• Upgrade Hugo to 0.112.3 and fix en title #1534 - preview https://deploy-preview-1534--docsydocs.netlify.app/docs/adding-content/shortcodes/#tabbed-panes: no longer works. Selecting "C++" doesn't change the code. Selecting "Kotlin" shows two panes.

After some investigation I found that tabpane is generating duplicate HTML element IDs for the tab pane HTML element. For proof, visit https://validator.w3.org/nu/?doc=https%3A%2F%2Fdeploy-preview-1534--docsydocs.netlify.app%2Fdocs%2Fadding-content%2Fshortcodes%2F: it confirms

Error: Duplicate ID tabs-1