Weird roslyn waring

never-not.cs

public interface IArgument
{
    /// <summary>
    /// The value of the argument sans quotes.
    /// </summary>
    string Value { get; }
}

public interface IArguments : IReadOnlyList<IArgument>, IArgument
{
  void Deconstruct(out IArgument first, out IArgument second, out IArgument third);
}

var (verb, mention, description) = messageContext.Arguments; // messageContext.Arguments implements IArguments

// Error CA1508 : 'mention.Value is "not"' is always 'false'. Remove or refactor the condition(s) to avoid dead code.
if (verb.Value is "is" && mention.Value is "not")
{
}

// However, this works fine:

if (verb is {Value: "is"} && mention is {Value: "not" })
{
}

I have these set in the .csproj

<TargetFramework>net5.0</TargetFramework>
<Nullable>enable</Nullable>
<LangVersion>9.0</LangVersion>

...
<PackageReference Include="Microsoft.CodeAnalysis.NetAnalyzers" Version="5.0.3">
      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
      <PrivateAssets>all</PrivateAssets>
    </PackageReference>