Please try <painful solution that is worse than bug>
The Galaxy S9 series has a bug where joining a WiFi access point doesn't turn off cellular data. As soon as I join a WiFi network, apps start transferring everything in their queue over their original cellular connection. This phone has changed my data usage from about 600MB/month to 3+ GB/month and it costs extra. It's trivial to reproduce this but Samsung Support can't be bothered to do anything but offer canned useless responses.
Why yes, I can solve the problem by performing a factory reset and never launching any apps. Hey, thanks.