I'm probably missing something but I thought in the past I could easily turn-off WiFi and not have the WiFi turn itself back on. Turning off the switch in advanced WiFi settings "Turn on WiFi automatically" seems to have no effect.
You may have an app or routine set that automatically turns it on. Some apps have control of Wi-Fi and can turn it on as needed or even when not needed by the app. Go Settings->Apps->Special Access->Wi-Fi Control and see if any app is allowed to auto turn it on. If you have Bixby Routines configured to perform certain actions by location, check that as well.
Thanks I'll take a look at the settings you suggested. The reason I posted it here was that this didn't happen before the Beta install and I haven't installed any apps then.
Please try the suggestions made by @Ekamek . If the issue persists, please send us an error report. We apologize for the inconvenience and appreciate your contribution to the beta program.
One UI Beta Team.
Well it appears the app that kept turning the WiFi back on was Bixby. Again never had this issue when running the old stable version of the U.I. so don't know if there was a base change in Bixby or some switch in the app was flipped on.