If you open your My Leviton app and your existing devices have disappeared, the most likely cause is the app is not correctly showing your Residence. The solution is to choose your Residence.
At the top of the app your Residence name should appear in the middle. If you only see a drop down area and no text, press to the left of the arrow:
Choose your Residence from the drop down list:
After choosing your Residence your devices will correctly return to your app.