The data plan may sometimes be used on some devices when Wi-Fi is connected. An annoying bug that can significantly increase 4G data consumption on Android Oreo.
Will Android Oreo risk putting our data plans at risk? In the US Reddit forum, a user explains that after updating his Pixel XL, he noticed a peak in the use of data traffic when using Wi-Fi, so that Wi-Fi would not completely take over 4G data when activated.
By visiting Android developer options, he noticed that the "Mobile data always enabled" feature was checked by default, which was not the case before the update. Unchecking this option has ended the peak of data consumptions.
A gbad memory of iOS 9
The bug does not seem to affect all the smartphones passed under Android Oreo. The Canadian site MobileSyrup, which found the information on Reddit, explains that the option was indeed deactivated by default on their Nexus 5X under Oreo. According to him, Google is aware of the bug that touches at least the Pixel XL and planned to remedy it in the next update.
In the meantime, to avoid having to overcome your data package a little too quickly and unintentionally, it is better to uncheck the option. To do this, go to Settings> System> About Phone. There, scroll down to the Android version number and tap it several times until the developer menu is unlocked. This is where you can deactivate the option "Mobile data always enabled".
This case is reminiscent of that of iOS 9 which offered a "Wi-Fi support" to overcome a bad Wi-Fi connection by using the cellular network. Some users then noticed a significant increase in their data plan.