Probably a user/setup issue, but unfurling is enabled and in the admin site the test works fine but nothing happens when posting on the form unless I open a private window.
JS isn’t related here and changes to the browser environment shouldn’t make a difference so certainly not a bug but very likely something else with the setup.
Where it's caching the old files before the 2.1 update it doesn't work when posting. But once posted from an uncached browser it shows correctly for all. The htaccess file was overwritten during the update so not caching from there.