NSToolbarItem manages its own style when isBordered is set to true (for example in your code is not manually adding a NSButton subview).
On the other hand, your SwiftUI code is manually adding a Button instead, and probably NSToolbarItem isn't smart enough to recognise the pattern and fall back to its internal NSButton subclass.
I don't know if there is a workaround, I would send a feedback of the issue and keep avoid NSHostingView for simple toolbar buttons, it doesn't like there isn't any advantage in SwiftUI for such things anyway.
Topic:
UI Frameworks
SubTopic:
SwiftUI
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