Probably an issue with the font dialog changing API in Qt 6 or something. Need to investigate.
Can confirm I can reproduce this issue as well.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/plasma-workspace/-/merge_requests/2993
Git commit 2d5cc97dcdc9a6267806201d50ea6154c5995009 by Nicolas Fella. Committed on 14/06/2023 at 10:16. Pushed by nicolasfella into branch 'master'. [applets/digitalclock] Add parent window to font dialog Otherwise it doesn't show M +1 -0 applets/digital-clock/package/contents/ui/configAppearance.qml https://invent.kde.org/plasma/plasma-workspace/-/commit/2d5cc97dcdc9a6267806201d50ea6154c5995009