Android 8.0 (Oreo) users may want to double check their data plans; a user named Unusual_Sauce on Reddit, among other users, is reporting that their mobile data allotment is being used up when their handset is connected to Wi-Fi. There are a few users in the same thread reporting that they do not have the issue, and there seems to be no pattern to who will have the problem and who won't based on what device they're using or any other conceivable factors. Google is reportedly aware of the issue and working on a fix, but for the time being, no mobile carriers have issued a statement on the problem, so customers who have blown through their data limit and wound up throttled or facing down a huge bill for additional data usage may be out in the cold.
For now, the only fix that anybody with the issue has found is to simply disable mobile data while on Wi-Fi. There is no setting to do just that, so users have to disable mobile data when getting onto a Wi-Fi connection, then disable it again when heading away from the connection site. Android normally handles passing a user back and forth between available Wi-Fi and cellular networks fairly well based on the strength of both networks, favoring Wi-Fi. This new bug shows users every sign that their device is connected to and using Wi-Fi, but eats from the data plan as if no Wi-Fi is present. Thus far, users have not concluded if mobile data and Wi-Fi are being used simultaneously, like the Download Boost mode found on Samsung devices, or if the device is using mobile data exclusively. Users can rest easy in the knowledge that their devices are not using Wi-Fi and reporting it as mobile data usage; data use measurement for carriers takes place on the server side.
As stated above, affected users can choose to either deal with the issue until it's fixed, as most with an unlimited plan likely will, or turn off mobile data whenever they're around a Wi-Fi connection. The only alternative is to simply downgrade from Android Oreo to Nougat (7.0) or a lower version. The issue was apparently not reported with earlier Developer Preview versions of Oreo, so those who have a compatible Nexus or Pixel, or were running an Oreo ROM built from one of those on another device can likely fall back to that and continue enjoying most Oreo features, while being safe from this bug until Google implements a fix.