The most recent monthly security update for Pixels was more than just security fixes. It was Google's second 'Pixel Drop,' with several new features and fixes, including a new option to enable the system-wide dark theme automatically at a specific time each day. However, the automatic mode doesn't seem to work with all apps.
There are a few reports on social media of the new automatic mode not working properly, with some apps completely ignoring the change in theme. Gboard, Contacts, and the dialer are among the affected applications.
The good news is that Google is aware of the issue, and a fix is on the way. "Thank you for bringing this issue to our attention. We are aware of this issue and a fix will start rolling out soon," an official Google account posted to Reddit. There are also reports that switching to the beta channel for Gboard fixes that app's issues with automatic themes.