Whenever ArcGIS Pro (or ArcMap) returns error messages there is a chance that the error is actually a link to a page like 030024: Solve returned a failure:
The Network Analyst solver could not determine a solution.
The solver could fail to compute a solution in many scenarios. Check
the messages returned by the solver ...
Total_Length seems to be the (more realistic) 3D length of the route, Shape_Length the planar length. I did the following test: Exporting the route - which has a Polyline ZM Geometry and calculated the 3D Length - In my case (starting and end poins directly on the line segment) the 3D length equals the Total_Length.