I'm having the same issue and it is very annoying and really degrades the end user experince.
I've got an iPhone SE, and ComboBox set to size 25 font. When I click on the combobox, the application is zooming in substantially. After you pick a selection, you have to click out of the combobox, and then two finger manually zoom out (without clicking on any other part of the application).
Same over here. I've got users who stop using PowerApps because of this annoying issue. Don't even understand why this zooms in.