Hello @Severin
I see you are using sonar.cs.roslyn.ignoreIssues=true which currently has a bug, and removes also the reference to the source generator you are using. See: The `GetAnalyzerSettings` task is removing existing code generators when `sonar.cs.roslyn.ignoreIssues` is set to true · Issue #1211 · SonarSource/sonar-scanner-msbuild · GitHub.
While waiting for the issue to be resolved, you can try to set the value of sonar.cs.roslyn.ignoreIssues to false as a temprorary workaround.
All the best,
Čaba