This is rust-lang/cargo#9863
When a feature of a dependency is enabled, which enables optional transitive dependencies, those transitive dependencies are not marked as conditional in any way. One cannot work out "On Linux this dependency wouldn't be present". This remains the case even when --filter-platform is used.
Ideally, either via --filter-platform or via metadata in the returned data structure, it should be clear that on a particular platform the dependency does not exist.
Note that the fact that the feature is conditiona is present in the package element for the root package, just the transitive dependency isn't marked as conditional.