Package manager problem with SwiftUI previews

I'm using the Glassfy package in my project. My project has an iOS and WatchOS app. I added Glassfy as a package dependency, but have only added the library to my iOS app.

I can compile and run both apps just fine but when I try to use one of the WatchOS app views in Canvas preview I get errors from the Glassfy libraries saying different properties defined in the files are not available on watchOS. These blocks of code that give errors are wrapped in: if TARGET_OS_IPHONE. Glassfy supports both iOS and WatchOS, but the preview seems to ignore the compile flag.

Why is this failing and how can I get it working? Thanks