The modz macro (called by the Modify Elevation command) applies a scale factor to only 1/3 of the dimensionality and you end up with a custom linestyle scale of 0.66666667. This is a known issue and the recommendation is to only use Modify Elevation on elements that do not use custom linestyles. But - if you must, it is pretty easy to correct the linestyle scale.
Correct the linestyle scale: Select all elements with custom linestyles and execute the command Modify Line Style Attributes. Use the Scale icon and check the box on the left, set the value=1.0, and check on Absolute.
Alternative 1: When working with a DTM basemap in InRoads – you could simply View Features… from InRoads in a 2D model in a DGN – that is probably the easiest. This model could be named 2D Basemap.
Alternative 2: If you already have a 3D basemap, try File>Export>2D… - this is what I would use to “flatten” a 3D basemap into 2D graphics for drafting. This literally flattens your existing file – all of the 3D models in the file are changed to 2D models, but the names are still the same. You should edit the model properties and remove -3D from the model names if it is there.
Alternative 3: If you want to just set elevations on a few elements – or if the elements have vertices that are at different elevations – you could use the Set Element Elevation command on the CivTools mdl. Load CivTool by typing into the key-in window: mdl load civtools, wait for the Civil/Site Tools toolbox to open.
Alternative 4: You can change the elevation of element points using the Element Properties – but this is very difficult when you have linestrings with multiple elevations on many vertices.