Not knowing your work flow or file background I have the following comments that may help you:
Is this file part of a sheet set? If it is and you placed the view onto the layout via the sheet set manager you may find the sheet set has control of the viewport.
If that is not the case and you created all this from the gound up, Eg created the viewport, drew the lines in modelspace, etc etc then it will most likely be a layering issue, HOWEVER if any of the objects in modlespace are set to annotative (usually related to blocks or text) the lack of an assigned scale will prohibit objects from being visible through viewports.
Unfortunately the drawing you provided is incredibly laggy on my machine, I would recommend a few admin clean up tasks
-1. purge the drawing
-2. create a blank drawing and copy all the objects from modelspace to the new clean drawing
-3. once in the new clean drawing right click the layout tab and select from template >> browse to the original file and select the layout to import..
-minimize the vertices on civil layers through the use of xrefs and blocks, avoid nested xrefs and nested blocks they make maintenance challenging
-if there are a large qty of splines you may experiment with converting them to polyline with less precision
Even though the file size you provided is 2.6meg it's still taking a lot of resources to handle it, there has to be some garbage buried in there, do the steps mentioned should have a positive effect on the performance.
Best Regards
-Tim C.
