I know how you feel, I was incredibly frustrated for months when they broke this function (it used to work just fine) but refused to admit it. I even ended up returning Ring cameras I’d bought but I’d had the doorbells too long for a refund. They have finally fixed this issue however (at least with the iOS app) but I won’t be buying any more Ring products. Here’s how to activate the feature:
Be sure you update your app, then on the home page of the app, tap the elipsis “…” in the top right corner of the device you want to pop up full screen, tap Settings, Device Settings, Alert Settings, Notification Preferences and turn off (yes, off) In-App Notifications. This will then show alerts full screen, but only when you are in the app.