Hi @boubcher, just taking a looking at this dataset myself I have been able to aggregate the features okay. Inspecting the output from the Aggregator, I can see that the fme_geometry = fme_aggregate and the fme_type = fme_line.
In your workspace, is the geometry changing after you have written out your features to PostGIS or Shapefile, or is it incorrect after you aggregate the features (and before you write it out)? If you are still having problems with your geometry you could try using the GeometryCoercer to reset the geometry type of the feature.
If possible, would you be able to share your workspace? If you're not comfortable sharing your workspace here there's always the option to upload it to the Safe Software Support FTP.