Apple released iOS 9 for supported devices just a couple of weeks ago and rolled out a few updates for its latest operating system three days ago as well. Just last week, the company claimed that iOS 9 is now on 50% of its devices, which according to Apple, made it the fastest rollout ever.
While the iOS 9.0.1 update fixed "an issue where some customers with a custom APN setup via a profile would lose cellular data", there appears to be one other setting that could eat through your data plan. This setting definitely isn't a bug but it could cause distress to users with limited data plans.
The new feature available in iOS 9 is called "Wi-Fi Assist" and it automatically turns on when you update to the latest version of the OS. This feature is particularly helpful when you're on a weak Wi-Fi network as it detects the strength of the Wi-Fi signal and then shifts you over to cellular data plan if you're experiencing buffering issues.
While this feature is helpful in places where you have limited Wi-Fi connectivity, it's probably not suitable for people with data caps as it may incur additional data charges. Many users over on Twitter have been reporting that their data usage has escalated from the usual 1 GB a month to 3 GB and in some cases, even as much as 7 GB.
Fortunately, there's an easy fix for this issue. Users who are inadvertently using extra data due to this feature can simply head over to Settings > Cellular and then scroll down to turn off Wi-Fi Assist. This is obviously not a bug and Apple is reportedly aware of the issue but turning off the feature is apparently the simplest solution as of now.