After further investigation, the issue is found to be a browser limitation. The progress area is updated correctly (you could observe the HTML elements and the network traffic with the developers' tools) but the UI is frozen. There is a bug report sent to Apple about this issue for iOS 7, but the issue still exists on iOS 8.1. If the RadProgressArea is put inside a RadAjaxPanel, the UI can be updated and the ProgressArea is visible during upload postback.