repro

UnitTest1.cs

using CommandLine;
using System;
using System.IO;
using Xunit;

namespace XUnitTestProject5
{
    public class CommandLineOptions
    {
        [Option('a', "aio", Required = false, Default = true, HelpText = "Use Linux AIO")]
        public bool UseAio { get; set; }
    }

    public static class ConsoleLineArgumentsParser
    {
        public static (bool isSuccess, CommandLineOptions commandLineOptions) ParseArguments(string[] args)
        {
            bool isSuccess = false;
            CommandLineOptions commandLineOptions = null;

            using (var parser = CreateParser())
            {
                parser
                    .ParseArguments<CommandLineOptions>(args)
                    .WithParsed(options => { commandLineOptions = options; isSuccess = true; })
                    .WithNotParsed(_ => isSuccess = false);
            }

            return (isSuccess, commandLineOptions);
        }

        private static Parser CreateParser()
            => new Parser(settings =>
            {
                settings.CaseInsensitiveEnumValues = true;
                settings.CaseSensitive = false;
                settings.EnableDashDash = true;
                settings.HelpWriter = Console.Out;
                settings.IgnoreUnknownArguments = true; // for args that we pass to Host.CreateDefaultBuilder()
                settings.MaximumDisplayWidth = GetMaximumDisplayWidth();
            });

        private static int GetMaximumDisplayWidth()
        {
            const int MinimumDisplayWidth = 80;

            try
            {
                return Math.Max(MinimumDisplayWidth, Console.WindowWidth);
            }
            catch (IOException)
            {
                return MinimumDisplayWidth;
            }
        }
    }

    public class ConsoleLineArgumentsParserTests
    {
        [Theory]
        [InlineData(true, "-a", "true")]
        [InlineData(true, "-a=true")]
        [InlineData(true, "--aio", "true")]
        [InlineData(false, "-a", "false")]
        [InlineData(false, "-a", "False")]
        [InlineData(false, "-a=false")]
        [InlineData(false, "--aio", "false")]
        [InlineData(false, "--aio", "False")]
        public void AioCanBeParsed(bool expectedAio, params string[] args)
        {
            (bool isSuccess, CommandLineOptions commandLineOptions) = ConsoleLineArgumentsParser.ParseArguments(args);

            Assert.True(isSuccess);
            Assert.Equal(expectedAio, commandLineOptions.UseAio);
        }
    }
}

XUnitTestProject5.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFrameworks>netcoreapp2.1;netcoreapp3.1</TargetFrameworks>

    <IsPackable>false</IsPackable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="CommandLineParser" Version="2.7.82" />
    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="16.2.0" />
    <PackageReference Include="xunit" Version="2.4.0" />
    <PackageReference Include="xunit.runner.visualstudio" Version="2.4.0" />
    <PackageReference Include="coverlet.collector" Version="1.0.1" />
  </ItemGroup>

</Project>