Hi,
When coming from GDML, which does not store visualisation attributes, ROOT invokes TGeoManager::SetDefaultColors(). This loops over volumes and assigns some colours to all volumes, but also makes transparent all volumes for which the density is less than 0.1. This is your case, since you have not defined materials in your file so ROOT assigned an automatic one with 0 density. What you can do before you draw:
gGeoManager->GetTopVolume()->SetTransparency(0);
also, to draw the original transparent volume, you should try “ogl” instead of “ogle”