Google has taken to their own Product Forums in the Pixel and Pixel XL section to notify vexed Android device owners that they are aware of recent issues with the Trusted Voice option turning off on its own, and are working on a fix. Trusted Voice is a Smart Lock option that lets a user unlock their phone just by saying OK Google, which means that saying OK Google to a phone that's affected by this bug while the screen is off will force a user to unlock the phone to do all but the simplest things. A Googler by the handle of Orrin has assured users that a fix is in the works and will be rolling out soon. For the time being, he left users with a workaround that should keep Trusted Voice running with no issues.
The issue manifests not in any particular part of how Trusted Voice works, but rather that the setting simply turns itself off at random. Users can set and improve their voice model like normal, but Trusted Voice will simply turn off at some point. Google's workaround should fix that, and allow users to keep Trusted Voice on. The workaround is fairly simple; all you have to do is go into your Google app's settings menu, where you'll find "Accounts and Privacy". From that screen, go to "Google Activity Controls", and ensure that "Voice and Audio Activity" is turned on. Do this for every account present on a device, and your setting for Trusted Voice should stick, though devices like the Nexus 6 that don't have Trusted Voice as an option at all won't be able to gain it through this workaround.
Google says that they will update the thread on Google's product forums when the fix begins to roll out. The issue is being posted about on the Google Product Forum for Pixel devices, but the fix will likely roll out as an integral Android update, perhaps alongside the February security update, which means that other devices that have been seeing the issue will benefit once the update reaches them, which will of course depend on their device, manufacturer, and carrier branding, if applicable.