When using simplebrowser and running e.g.:

sleep 1 && xdotool type 🤔

and then focusing the search field here on Jira, 樂 is typed instead (that is, unicode codepoint U+1F914 turns into U+F914).

This was fixed for Qt itself here: QTBUG-72776 / qxkbcommon: UCS-4-encode surrogate characters in QKeyEvent::key() (I9d1f4e34) · Gerrit Code Review

And it indeed works fine in e.g. the URL bar, but not in web contents. It does, however, work fine in Chromium.