macOS Sonoma is here and Things is ready to go! Widgets are now interactive, on your desktop, and available in two new formats → https://culturedcode.com/things/blog/
Things Blog - Cultured Code

@things Added a new widget in Sonoma, but it's not interactive. A checkmark appears for a second or two, then disappears. The task remains uncompleted. What’s up?
@mikkoh @things Same here. Any fix for this?
@datboi @things For me, restarting the Mac fixed it.
Troubleshooting Widgets

If you’re having trouble with Things’ widgets, please check out the tips on this page.

Things Support