How to make FSI work under NET5 and make stupid stackoverflow message "Title cannot contain ..." shut up?

I am migrating a fairly small F# project from Net Framework to NET5. The migration was very easy and everything works including the tests.

However, when I run some scripts I am now getting the following error:

Microsoft (R) F# Interactive version 11.0.0.0 for F# 5.0
Copyright (c) Microsoft Corporation. All Rights Reserved.

For help type #help;;

> 
[Loading C:\Users\kkkma\AppData\Local\Temp\nuget\28772--091a5e4a-2a27-4d4d-9891-2b58055c5317\Project.fsproj.fsx
 Loading C:\GitHub\Berreman\Berreman\Analytics\Examples\References.fsx]
namespace FSI_0002.Project

namespace FSI_0002

[Loading C:\Users\kkkma\AppData\Local\Temp\nuget\28772--091a5e4a-2a27-4d4d-9891-2b58055c5317\Project.fsproj.fsx]
namespace FSI_0003.Project

Binding session to 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\System.Runtime.dll'...
Binding session to 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\System.Runtime.Numerics.dll'...
System.TypeLoadException: Could not load type 'System.ICloneable' from assembly 'System.Runtime, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'.
   at Berreman.MathNetNumericsMath.complexDiagonalMatrix(Int32 n, Complex e)
   at Berreman.Geometry.complexIdentityMatrix(Int32 n) in C:\GitHub\Berreman\Berreman\Berreman\Geometry.fs:line 17
   at Berreman.Geometry.ComplexMatrix3x3.get_identity() in C:\GitHub\Berreman\Berreman\Berreman\Geometry.fs:line 307
   at Berreman.MaterialProperties.Eps.fromRefractionIndex(RefractionIndex _arg1) in C:\GitHub\Berreman\Berreman\Berreman\MaterialProperties.fs:line 70
   at FSI_0004.opticalProperties(RefractionIndex refractionIndex) in C:\GitHub\Berreman\Berreman\Analytics\Examples\Glass_01.fsx:line 51
   at FSI_0004.getGlassInfo(Boolean useThickPlate, RefractionIndexThickness nh1, FSharpOption`1 nh2Opt, IncidentLightInfo light) in C:\GitHub\Berreman\Berreman\Analytics\Examples\Glass_01.fsx:line 61
   at <StartupCode$FSI_0004>.$FSI_0004.main@() in C:\GitHub\Berreman\Berreman\Analytics\Examples\Glass_01.fsx:line 98
Stopped due to error
> 

When I examine Project.fsproj.resolvedReferences.paths from C:\Users\kkkma\AppData\Local\Temp\nuget\28772--091a5e4a-2a27-4d4d-9891-2b58055c5317 (and that's the folder created by FSI when I run the script) I see that it starts from C:\Users\kkkma\.nuget\packages\microsoft.netframework.referenceassemblies.net48\1.0.0\build\.NETFramework\v4.8\Facades\System.Runtime.InteropServices.RuntimeInformation.dll and it also has some other Net Framework resolved references. Which makes me believe that FSI somehow "decides" to use Net Framework instead of NET 5.

The file fsi.runtimeconfig.json contains correct information:

{
  "runtimeOptions": {
    "tfm": "net5.0",
    "framework": {
      "name": "Microsoft.NETCore.App",
      "version": "5.0.0"
    }
  }
}

Does anyone have an idea how to force FSI pick up NET 5 instead of Net Framework?