Consider the following class declaration:

type C<'T when 'T: unmanaged> = class end

When decompiled, it results in the following C# class declaration:

[Serializable]
[CompilationMapping(SourceConstructFlags.ObjectType)]
public class C<T>
{
}

The unmanaged constraint has disappeared. 'T does not have any constraints at all.

Because the unmanaged constrained appeared first in F#, it looks like it is stored in an F#-specific way (the SignatureData I guess?).

This is how it sould be represented to be recognized by C# (and F# for backwards compatibility, some code omitted for brevity):

namespace System.Runtime.CompilerServices

// This is a compiler-generated attribute. It has two other attributes,
// CompilerGenerated and Embedded, the latter of which is also compiler-generated.
type internal IsUnmanagedAttribute() = inherit System.Attribute()

namespace MyLibrary

type C<[<IsUnmanaged>] 'T when 'T: struct, unmanaged> = class end