For those that don't want to visit the external link...
To get the "Search on TrueAchievements" option back when viewing an achievement on Xbox One:
1.
Get/install the app from the Microsoft Store* (via Xbox.com: only if remote installations are allowed on your Xbox One console)
2.
Load the TrueAchievements app for the first time (required after a clean install or factory reset)
* To allow remote installations on Xbox One, use the Settings app, then proceed to System -> Updates & downloads and make sure the
Allow remote installations option is enabled. Alternatively, you can also reinstall the app from My games & apps on Xbox One (Apps -> Ready to install).