I am thinking it has to do with the layer the block is placed on. I suspect your lines are taking on the line type that is setup for the layer.
You beat me to it.
Any block pulled in should end up on layer -none- when you check the layer status in the property manager. If you are however on any other layer then -Per Standard- at the time of you dropping the block on your drawing then it will be changed to the settings of the layer you are using.
Changing the layer of the block to -none- or -Per Standard- should make it look as you designed it.
Change the layer that the block is placed on to 'None' and it will show the symbology of the underlying elements. Microstation does this to. as a former AutoCAD designer, it drove me nuts trying to figure out how to get a cell to display correctly.
Thank you all for your quick response.
John - Carried out your solution and worked a treat!
Keep the up the good work guys...