I have had this happen a few times myself, when you select a poyline the elevation displays as a strange nonsensical number.
Instead follow this workflow.
Explode from Featurelines to 3d polylines.
Convert from 3d Polylines to 2d poylines.
Now explode these polylines to lines.
use flatten or use the properties and change Start/End Z levels to zero.
Use Pedit Multiple to join all lines together again.
Mike Evans
Civil3D 2022 English
Windows 7 Professional 64-bit
Intel(R) Core(TM) i7-3820 CPU @ 3.60GHz (8 CPUs), ~4.0GHz With 32768MB RAM, AMD FirePro V4900, Dedicated Memory: 984 MB, Shared Memory: 814 MB