This is ho all outlines work, it's not specific to the non-native-theme, they're just covered by other adjacent elements because the non-native theme has a bigger outline. Same happens in Safari for reference (though there inputs have margins by default so you need to remove that first). Adding margins by default would "fix" this test-case, but changing the default margin of the input is pretty risky for little benefit imo.
Bug 1708560 Comment 3 Edit History
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This is how all outlines work, it's not specific to the non-native-theme, they're just covered by other adjacent elements because the non-native theme has a bigger outline. Same happens in Safari for reference (though there inputs have margins by default so you need to remove that first). Adding margins by default would "fix" this test-case, but changing the default margin of the input is pretty risky for little benefit imo.