Unable to add reference to netstandard through Visual Studio UI

[workgroupengineering]

How Reproduce:

1. Create multi target C# Project like this

   <Project Sdk="MSBuild.Sdk.Extras" ToolsVersion="15.0">
     <PropertyGroup>
       <TargetFrameworks>netstandard2.1;xamarinmac20;xamarinios10;MonoAndroid11.0;MonoAndroid10.0</TargetFrameworks>
       <LangVersion>latest</LangVersion>
     </PropertyGroup>
   
     <ItemGroup Condition="'$(TargetFramework)' == 'netstandard2.0'">
       <PackageReference Include="System.Buffers" Version="4.5.1" />
       <PackageReference Include="System.Memory" Version="4.5.4" />
     </ItemGroup>
   
     <ItemGroup>
       <PackageReference Include="Microsoft.IO.RecyclableMemoryStream" Version="2.0.0" />    
     </ItemGroup>
   </Project>

2. Try to add reference to netstandard through Visual Studio UI

I have already posted the issue at https://developercommunity.visualstudio.com/, but they suggest opening it here.