Don't use the app's normal save format if you want to keep your layers - it's going to merge everything with no way back. What nobody mentions is that you have to export to a project format before closing, not just hit "save". In Express, when you go file → export, look for the option to save as PSD or as the app's project format (it usually shows up as "save backup" or something similar depending on your version). That keeps the layers intact for your next session.
The problem is that Express is designed for quick mobile editing, not complex projects. If you save as a regular image (JPEG, PNG), obviously it loses all the layer information because those formats don't support them. But if the app has a native project option, always use it. Either way, if your workflow is serious and you need stable layers, the app is pretty limited - consider trying Krita or Ibis Paint X on Android, they handle multi-layer projects better.