A single device in a scene should work. Similarly a Xiaomi button should be able to activate a single device, not just a scene. There is something else happening. I don’t know what. Everything that follows is just background information about buttons …
In the ‘Classic’ environment, the button capability only defined ‘pressed’ and ‘held’, so if you wanted to handle things like multiple button devices or double clicks the typical approach was to add data such as a button number. So if the button was ‘pressed’ you could additionally check the button number to get more precise information (e.g button 1 means it was just a single click, button 2 a double click). That requires the app handling automations to know what is going on. WebCoRE knows this. I believe Smart Lighting does too. The Automation creator doesn’t. However it will still respond to a simple single press just fine.
In the ‘new’ environment, the button capability has been extended to understand multiple clicks. The automation creator knows this. I haven’t seen a button device handler that does and I don’t think Smart Lighting does either. WebCoRE can be made to understand by piston trickery.
The Xiaomi Button handler invents a ‘buttonStatus’ attribute for display purposes in the Classic app, so you can see if it was double clicked, for example, and you could also use it in webCoRE if you wanted. The new app can’t deal with that at the moment as custom capabilities haven’t been released yet.
The new app does understand composite buttons though. So if the device handler defines component child devices, it know how to work with them. The handler for IKEA buttons and remotes covers both bases by using button numbers and child components. The Xiaomi button handler doesn’t, probably because none of this stuff has been documented.
Generally speaking the issue with custom handlers and the new app is to do with the user interface being defined differently, and the odd quirk here and there in the settings. If you don’t need to use the new app to control a device, there is nothing to worry about.