It's also possible that the specific linetype you're trying to import isn't supported.
I think it defaults to the linetype of the layer and not the object on the layer.
Hopefully they can address this soon.
I believe that by default, Impression assigns objects properties based on the layer.
You can override that inside Impression, but a better option is to prepare drawingswhile in Autocad FOR Impression.
It's better to move (or copy) objects that should appear as dashed to a separate layer, so you could just apply a dashed style to that layer once inside Impression and affect all the objects in that layer at once.