I have some issues with the styling of StackLayout on iOS and Android when using padding. To isolate the issue I've created a new application with the following template:
tns create layout --tsc

Here's the changed code of the main-page.xml:

<Page xmlns="http://schemas.nativescript.org/tns.xsd" navigatingTo="navigatingTo" class="page" actionBarHidden="true">
    <StackLayout style="padding: 20" verticalAlignment="center" backgroundColor="red">
       <StackLayout height="60%" backgroundColor="green" borderRadius="10">
       </StackLayout>
    </StackLayout>
</Page>

And here's the output in both platforms:

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