Hi there, @plenartFKH, welcome to the community!
The v3 tasks target the latest version of the scanner for .NET (v9) which includes “multi language analysis”. This automatically discovers files in the repo that are not part of the project but that we can analyze. You can read more about it in the Scanner for .NET v8 announcement.
For monorepo, it is however not suitable and you should disable that behavior using the sonar.scanner.scanAll=false property.
Denis