Wrong syntax highlighting when using `await using`

When creating a simple file that uses the await using construct, syntax highlighting is broken below the point where it is used. For example, keywords like public are no longer displayed in dark blue, see screenshot below. This is a simple example code:

using System;
using System.Threading.Tasks;

namespace vscode_await_using_bug
{
    class Program : IAsyncDisposable
    {
        static async Task Main(string[] args)
        {
            await using (var v = new Program())
            {
                Foo();
            }
        }

        public async ValueTask DisposeAsync()
        {
            await Task.Delay(50);
            Console.WriteLine("Byebye");
        }

        public static void Foo()
        {
            Console.WriteLine("Hello World!");
        }
    }
}

Environment data

dotnet --info output: .NET Core SDK (reflecting any global.json): Version: 3.1.301 Commit: 7feb845744

Runtime Environment: OS Name: ubuntu OS Version: 20.04 OS Platform: Linux RID: linux-x64 Base Path: /usr/share/dotnet/sdk/3.1.301/

Host (useful for support): Version: 3.1.5 Commit: 65cd789777

.NET Core SDKs installed: 3.1.301 [/usr/share/dotnet/sdk]

.NET Core runtimes installed: Microsoft.AspNetCore.App 3.1.5 [/usr/share/dotnet/shared/Microsoft.AspNetCore.App] Microsoft.NETCore.App 3.1.5 [/usr/share/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET Core runtimes or SDKs: https://aka.ms/dotnet-download

VS Code version: 1.47.1

C# Extension version: 1.22.1

OmniSharp log

Starting OmniSharp server at 7/16/2020, 5:33:04 PM Target: /home/gidoca/src/vscode-await-using-bug

OmniSharp server started. Path: /home/gidoca/.vscode/extensions/ms-dotnettools.csharp-1.22.1/.omnisharp/1.35.3/run PID: 264088

    Starting OmniSharp on ubuntu 20.4 (x64)

    DotNetPath set to dotnet

    Located 1 MSBuild instance(s)
        1: StandAlone 16.4 - "/home/gidoca/.vscode/extensions/ms-dotnettools.csharp-1.22.1/.omnisharp/1.35.3/omnisharp/.msbuild/Current/Bin"

    MSBUILD_EXE_PATH environment variable set to '/home/gidoca/.vscode/extensions/ms-dotnettools.csharp-1.22.1/.omnisharp/1.35.3/omnisharp/.msbuild/Current/Bin/MSBuild.dll'

    Registered MSBuild instance: StandAlone 16.4 - "/home/gidoca/.vscode/extensions/ms-dotnettools.csharp-1.22.1/.omnisharp/1.35.3/omnisharp/.msbuild/Current/Bin"
        CscToolExe = csc.exe
        MSBuildToolsPath = /home/gidoca/.vscode/extensions/ms-dotnettools.csharp-1.22.1/.omnisharp/1.35.3/omnisharp/.msbuild/Current/Bin
        CscToolPath = /home/gidoca/.vscode/extensions/ms-dotnettools.csharp-1.22.1/.omnisharp/1.35.3/omnisharp/.msbuild/Current/Bin/Roslyn
        BypassFrameworkInstallChecks = true
        MSBuildExtensionsPath = /home/gidoca/.vscode/extensions/ms-dotnettools.csharp-1.22.1/.omnisharp/1.35.3/omnisharp/.msbuild

    Detecting Cake files in '/home/gidoca/src/vscode-await-using-bug'.

    Could not find any Cake files

    No solution files found in '/home/gidoca/src/vscode-await-using-bug'

    Detecting CSX files in '/home/gidoca/src/vscode-await-using-bug'.

    Could not find any CSX files

    Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.CSharpFormattingWorkspaceOptionsProvider, Order: 0

    Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.RenameWorkspaceOptionsProvider, Order: 100

    Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.ImplementTypeWorkspaceOptionsProvider, Order: 110

    Invoking Workspace Options Provider: OmniSharp.Roslyn.CSharp.Services.BlockStructureWorkspaceOptionsProvider, Order: 140

    Configuration finished.

    Omnisharp server running using Stdio at location '/home/gidoca/src/vscode-await-using-bug' on host 263959.

Steps to reproduce

Create a file named Program.cs with the content above.

Expected behavior

The syntax highlight is the same as synchronous using.

Actual behavior

The syntax highlighting no longer marks keywords correctly. using is yellow, public and static are light blue.

dotnet / vscode-csharp