In 2020, SmartThings completed the transition to a new app and cloud architecture. The change is an improved user experience that spans a broader range of Samsung products. While most customers have transitioned to the new app without any issues, technical services has received calls stating devices may not appear in the new app. For customers that experience this issue the recommendation is to default your Leviton devices and reenroll them to the SmartThings hub using the new app.
The article: How do I add my Z-Wave Switch or Dimmer to my Samsung SmartThings System? will guide you through the pairing process.
Samsung has a Frequently Asked Questions addressing the transition to the new app here.