Nullability in generated code does not match

[denys-volodarskyi]

public partial class TheClass : ReactiveUI.ReactiveObject
{
    public Func<string?, string?>? Validation { get; }
}

generates

public static IObservable<global::System.Func<string, string>?> WhenAnyValidation(this global::TheClass source)
{
    if (source is null) throw new ArgumentNullException(nameof(source));

    return new PropertyObserver<global::TheClass, global::System.Func<string, string>?>(
        source,
        "Validation",
        () => source.Validation);
}

output

Warning CS8619 : Nullability of reference types in value of type 'Func<string?, string?>' doesn't match target type 'Func<string, string>'