How to debug Ember addon with breakpoints in VS Code

Am trying to get breakpoints to work in VS Code with an Ember addon (version 3.18). Have tried launch.json as:

{
"version": "0.2.0",
"configurations": [
    {
        "type": "edge",
        "request": "launch",
        "name": "Launch Edge against localhost",
        "port": 9222,
        "runtimeArgs": [ "--remote-debugging-port=9222" ],
        "url": "http://localhost:4200",
        "sourceMapPathOverrides": {
            "dummy/*": "${workspaceFolder}/tests/dummy/app/*",
            "test-addon/*": "${workspaceFolder}/addon/*",
        }
    }
]}

It works fine for setting breakpoints in files in the dummy test app. But when setting a breakpoint in files within the addon folder, VSCode says "Breakpoint set but not yet bound". How can this be fixed? I assume the 2nd sourcemap path override is wrong?