I have the same problem (just started last week) on S8+ . Chatted with xfinity tech who had me check a couple of settings in Settings/Connections/Wifi/Advanced. Specifically, check that "switch to mobile data" is off (originally she told me to turn off smart network switch but that setting is not on my S8+) and enable ''turn on wifi automatically.''
She asked me to monitor for a day and see if issue persists (it does) and if it does contact Samsung since it was their update that caused the problem. Interestingly, my wife and I have exactly the same phone, the same (latest installed Jan 30) Samsung update, and the same version (188.8.131.52) of the "Xfinity WiFi Settings" app and her phone is NOT having this issue.
When it occurred again yesterday, I went to "Xfinity Wifi Settings" app and turned off "Allow app to control Wi-Fi." This morning, I saw that the app had re-enabled that setting (what?!!!) and it turned off my WiFi a few minutes after I started using the phone this morning. Now I'm going to try a force stop but I don't have much hope for that working.
Agreed. I guess it would be like Adobe telling their users to call Microsoft when a Windows update causes problems with Photoshop, when it would be Adobe's responsibility to check/verify that all their drivers etc. would still work after the update.