Ionic Serve is working correctly, but Ionic build is not working correctly. Cordova does not execute in the browser, Ionic build uses Cordova which runs on your phone.
The issue is most likely in cordova packages.
Ionic cordova plugins , to list down the plugin list.
Also note, for building iOS apps, you now need the inAppBrowser version 3.2.0 from 30th April’20
I updated doc here https://gist.github.com/MrVibe/3beb7c429d34eb55f0bc64c4ddbda816