Some libraries as HQ Plants 1 & 2 or Presets, include Standard materials, which are easily converted to Octane. This is also the default selection of the Library Browser, when used in "Automatic" mode and the render engine is not recognized. Instead, HD Flowers includes only VRay and Mental Ray materials.
You would try to change manually the library material selector, from Automatic to VRay or Mental Ray, to see if there is some difference. In any case, the result depends of how Octane converts these type of materials.