reportXmlFile value is " D:\Repos\<redacted>\<redacted>\<redacted>.ViewModel.Test\TestResults\dc51ad10-1170-46f2-8717-e0be4990e439\coverage.cobertura.xml". I did not notice the preceding spaces and this seems to have caused the weird behaviour, though I'm not sure why. Trimming the input string to remove the two leading spaces has the FullName property returning a complete path now.