Rider on Fedora 35 won't show netstandard

Problem description

I want to us source generators and therefore need a class library that targets netstandard2.0. Unfortunately I can't select it in the JetBrains Rider framework dropdown (see screenshot below).

Environment details

Rider version: 2021.3.2 (Build: #RD-213.6461.51)

dotnet --info:

.NET SDK (gemäß "global.json"): Version: 6.0.100 Commit:
9e8b04bbff

Laufzeitumgebung: OS Name: fedora OS Version: 35 OS Platform: Linux RID: fedora.35-x64 Base Path:
/usr/lib64/dotnet/sdk/6.0.100/

Host (useful for support): Version: 6.0.0 Commit: 4822e3c3aa

.NET SDKs installed: 3.1.118 [/usr/lib64/dotnet/sdk] 5.0.206 [/usr/lib64/dotnet/sdk] 6.0.100 [/usr/lib64/dotnet/sdk]

.NET runtimes installed: Microsoft.AspNetCore.App 3.1.18 [/usr/lib64/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 5.0.9 [/usr/lib64/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 6.0.0 [/usr/lib64/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 3.1.18 [/usr/lib64/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 5.0.9 [/usr/lib64/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 6.0.0 [/usr/lib64/dotnet/shared/Microsoft.NETCore.App]

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

Framework selection in Rider: Framework dropdown greyed out and set to net6.0

Question

How can I get the framework-selection to let me select netstandard2.0 (and netstandard2.1)?

If you need any other info I'm more then willing to provide it. Thanks in advance.

Edit

After I have created a project with the preselected net6.0 framework selection my csproj looks like this:

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

    <PropertyGroup>
        <TargetFramework>net6.0</TargetFramework>
        <ImplicitUsings>enable</ImplicitUsings>
        <Nullable>enable</Nullable>
    </PropertyGroup>

</Project>

I can manually change the <TargetFramework> value and everything works (<TargetFramework>netstandard2.0</TargetFramework>), but I just find it strange that I can't use the dropdown in project creation directly.

Solution

It is a bug that has been reported to JetBrains team already: https://youtrack.jetbrains.com/issue/RIDER-70004

As a workaround, following can be done:

Right click on the solution and select Manage .NET SDK
Select older SDK (e.g. 3.1) and latestMinor as Roll-forward policy
Create new project for older framework
Open Manage .NET SDK and set Roll-forward policy to the state it was before