Such is the case with the recently released Android 8.0 Oreo update, in which some users are reporting that despite WiFi being turned on, their devices are still consuming mobile data. In a post on Reddit, some users are reporting that after the Oreo update, their devices are still using mobile data despite the fact that WiFi is enabled, and usually with WiFi enabled (and connected), it takes precedent over mobile data.
The good news is that Google appears to be aware of the issue and are working on a fix. The post reads, “After updating to Oreo last night, I received a huge spike in data usage, all the while being connected to WiFi. I contacted support and was told that they are aware of the issue and are working on a fix. In the meantime I have turned off mobile data while at home, so only WiFi is being used.”
This sounds similar to an issue that Apple had with the iOS feature WiFi Assist, in which some users were reporting that the feature actually caused them to blow through their data cap, and in some cases lead to overages in the thousands of dollars. That being said, the Oreo update is mostly limited to Nexus and Pixel devices at the moment, so hopefully Google will fix this before the update starts making its rounds to non-Google devices.