Hi,
Generating the report from a cobertura.xml file ends up with some classes missing, but using opencover works fine.
opencover report:

cobertura report:

The commands I used:
dotnet test /p:CollectCoverage=true /p:CoverletOutputFormat=cobertura /p:Exclude=\"[xunit*]*\"
dotnet reportgenerator -reports:./coverage.cobertura.xml -reporttypes:HTML -targetdir:./report/cobertura
Copies of the reports:
https://likasem.blob.core.windows.net/issues/coverage.cobertura.xml
https://likasem.blob.core.windows.net/issues/coverage.opencover.xml
Thanks for the great issue (with sample files attached).
I fixed that behavior. The next release will contain the change.