Hi @Rob_Young,
The Scanner for MSBuild requires unique identifiers for each analyzed project to be able to consistently match the analysis results and the SonarQube data. The <ProjectGuid> element is (was) a very good candidate for uniqueness because Visual Studio automatically assigns it when you create a new project. The .NET Core projects do not have <ProjectGuid> element by default and that’s why the analysis works only if you analyze the solution, which silently “assigns” unique GUIDs for the projects.
Other way to solve the problem is to add unique <ProjectGuid>GUID</ProjectGuid> elements in each project. This way you will be able to analyze separate projects without a solution.
This limitation is mentioned in the documentation.
Cheers,
Valeri