Custom IL SDK (similar language targets file to C#) doesn't show compile items inside Visual Studio

In our custom IL SDK we define these properties: https://github.com/dotnet/runtime/blob/891c6d36324540e1cb87586792d81cb22ff11c66/src/coreclr/.nuget/Microsoft.NET.Sdk.IL/targets/Microsoft.NET.Sdk.IL.targets#L15-L19.

Even though those are set, and the project file that uses the IL SDK even defines the compile item (https://github.com/dotnet/runtime/blob/891c6d36324540e1cb87586792d81cb22ff11c66/src/libraries/System.Runtime.CompilerServices.Unsafe/src/System.Runtime.CompilerServices.Unsafe.ilproj#L22-L24), Visual Studio doesn't show the item and only displays it when toggline "showing all files".

How does CPS/VS determine which items to show in the project tree. Are we missing a capability?

cc @drewnoakes