I executed "npm start" and "./mvnw" from my jHipster project folder on the different consoles.
Why contents on the http://localhost:9000 and http://localhost:8080 are different? http://localhost:8080 shows more old version of content.
This happens when you do a prod build, it puts the frontend files on target folder and package all in a war/jar. When accessing on port 8080, you are fetching that packaged frontend inside the war/jar, while for port 9000, its proxying the stuff you started with npm start. Do a ./mvnw clean or simply remove target folder.
Also you can compile frontend to have that files updated.
When using Angular 2+ if you need to do a clean run with webpack compilation enabled for dev profile you can pass the webpack param as below
./mvnw -Pdev,webpack
This way you will have updated the served content on 8080 port.