I´m suffering of this too, so I´ll add some more data.
It only happens in iPhone´s, multiple versions. At first i thought it happened only in screens with RG´s, but I´ve checked that it happen in almost all screens. Only thing I could check as a pattern is the existance of a reusable in the screen, in the footer and in the screen below.
In my case: Footer is a reusable (type Group) placed in a Floating Group in every screen, fixed left and bottom. In other versions, it was the reusable itself that was a floating group.
My footer have some other groups, some with negative position (hidden) as menu´s and chat screens that get positioned in the screen above the menu (as footer is, of course, in the bottom of the screen).
When the user scrollup or down, this bug happens.
Yes, there is only one footer in the screen.