I also get this on my Android phone (Samsung S8) when using the new SmartThings app. I have to manually force a network change notification by going into airplane mode and back for the new app to notice the network. The Classic SmartThings app doesn’t ever get this problem.
For me, it appears to happen as I travel home in the car, on LTE service. There is a dead zone near my house where there is no service. It seem like the new app latches onto the fact there is no network up until my phone switches to WiFi at the house. The funny thing is as I pull onto my driveway, I also get weak LTE service.
Either way, I just chalk it up to the crappy new SmartThings app.