A bug discovered in Android 8.0 Oreo spends users’ mobile data allowance, even when the phone’s mobile WiFi connection is enabled. Google is aware of the issue and is working on a fix, a Google spokesperson told Bleeping Computer. Android Oreo users who are used to enabling both Internet connection types should disable mobile data and use one connection at a time to avoid extra costs on their mobile plan. Users on other types of devices might have to update manually once a patch is out, as most mobile carriers are quite late on delivering Android updates.
Source: https://www.bleepingcomputer.com/news/mobile/android-oreo-bug-bypasses-wifi-to-use-mobile-data-and-incur-extra-costs/