I have a flat ActionBar flat: true in CSS and a ScrollView inside of a GridLayout. The view initially renders with the ScrollView underneath the ActionBar. A margin-top on the GridLayout (top level view) worked fine with the default, translucent ActionBar.
This is in NativeScript 4.2.4 and NativeScript Angular 6.1.10.
EDIT: The issue appears to be related with some delayed rendering. This seems to happen when the previous screen has the ActionBar hidden.
I found a suggestion that says to remove the transition effect between pages that do and don't have the ActionBar visible.
https://github.com/NativeScript/nativescript-angular/issues/1377