This behavior is as designed. The feature datasets and feature classes for network analysis layers use automatically-generated unique names. In previous releases, this was done using a numbering scheme, but this was changed later on to a string of alphanumeric characters.
When writing scripts or automating workflows in a different way, you should not try to access the network analysis layer’s sublayers directly by their data paths, as this is not a supported workflow and the naming convention is not static or guaranteed.
For ModelBuilder, if you need to access a sublayer of a network analysis layer, you can use the Select Data utility.
If you need to access the network analysis layer later, after the model has finished, you should use the Save To Layer File tool to save the layer itself to disk as a .lyrx file.