Project adding to solution folder doesn't work

Dotnet version: _11.0.100-preview.2.26159.112_
I have such file structure in my project:
.
├── Directory.Build.props
├── Directory.Build.targets
├── Directory.Packages.props
├── dotnet-tools.json
├── FileExchanger.slnx
├── global.json
├── nuget.config
└── src
├── FileExchanger
│   ├── appsettings.Development.json
│   ├── appsettings.json
│   ├── Controllers
│   ├── FileExchanger.csproj
│   ├── Models
│   ├── Program.cs
│   ├── Properties
│   └── Services
└── FileExchanger.Test
├── FileExchanger.Test.csproj
   ├── Infrastructure
   └── Tests

Content in FileExchanger.slnx:
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Then executed a command: "dotnet sln FileExchanger.slnx add src/FileExchanger.Test/FileExchanger.Test.csproj".
It said:
Project `src/FileExchanger.Test/FileExchanger.Test.csproj` added to the solution.
Solution folder 'src' already contains a project with the filename 'FileExchanger.csproj'.
But when I tried to "more FileExchanger.slnx" again there is nothing changed:

Even "dotnet sln FileExchanger.slnx add src/FileExchanger.Test/FileExchanger.Test.csproj --solution-folder src" returned the same answer:
_Project `src/FileExchanger.Test/FileExchanger.Test.csproj` added to the solution.
Solution folder 'src' already contains a project with the filename 'FileExchanger.csproj'._

_more FileExchanger.slnx

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