I had the same problem, you must config the proxy on ionic json file to allow works from web browser, for android or iphone devices that problem CORS doesn't happens
I'm developing an application ionic for a magento 2 website & get same issue when run app on website by command : ionic serve. I fixed it by fixing htaccess file and it works fine. But seem that this problem will not happen after build to real app on device.
By the way Now I'm using Token-based authentication to use web service. I get an access token at first login and save it in my app for next time. It's working fine.