The problem you are seeing is caused by @ionic/app-scripts which builds your app from /src to /wwwbefore ionic serve then displays it. Be using npm run ionic:serve you only display the app directly from /www, but your changes in TypeScript code in /src will not get reflected in /www.
Did you open an issue with Ionic CLI or app-scripts about the bug you are getting?
If users struggle enough, they take the initiative and report the errors as Github issues… especially after being told that this is the desired way to report problems and bugs.