I’d never had issues with the SmartThings App on my iPhone 8 Plus until upgrading to IOS 13.3.1.
Anyway, long story short, and I’m not sure who’s fault it is, but removed ST App v188.8.131.525, rebooted the iPhone, reinstalled the ST App (not classic) and bingo - all of the devices are working again. I mustn’t have purged the app data because there was no authentication requests and all the devices and layout are just as before I’d removed the app.
The failure symptoms were - all sensors in the app greyed out, and Leak Sensors timed out complaining about “network issues”, but only on my iPhone, iPad working as normal albeit on IOS 13.3 (NOT 13.3.1) and running ST App v1.6.42-350.
Hope this helps someone.