Hey there,
before opening a github issue i want to make sure others can confirm my problem.
I am using ionic-angular 3.5.2 and app-scripts 2.0.2.
Everything works fine with ionic serve and dev builds.
If i build a production version of the app, some styling is lost.
e.g. the ios-label and md-label classes are not in the main.css and the styling of every list in my app is broken.
Been running into this as well. Any update?
Can confirm, running 3.5.2 and scripts 2.0.2
I downgraded to 1.3.12 for the app scripts. Got my --prod builds working again, and works fine with the new ionic 3.5.3. Not a solution, but I can ship 
yeah but i want to use the new app scripts, because of the additional “vendor.js” to speed up development ;).
Probably unrelated, but 3.5.3 was insta-released to fix a nav bug in 3.5.2. So if you guys are still on 3.5.2, you might want to update just for that.
yeah… unrelated… i could update the topic title to 3.5.3, but like you said … they changed nothing. maybe i should edit the title to app-scripts 2.x instead a specific ionic version.
it is working for me with the latest release of app scripts, cli and ionic-angular

Which versions do you use?
Still happening here… 
ionic-angular: 3.6.0
app-scripts: 2.1.4
ionic-cli: 3.9.2