I’m experiencing this problem as well, on several (different) Apple devices, all with 11.3 iOS versions. It happens in both Safari and Chrome, as well as in the Axure app, and when placing the prototype as an “app” on the homescreen.
The workaround mentioned here works, sort of, for mobile, but when viewing the prototype on desktop (I use both), a sticky tabbar is also all the way at the bottom of the screen.
I have usability tests coming up, for which I now need to rework all of my prototypes… help!