When profiling a continuous line across a 3D surface using the Profile Graph tool in the layout, the tool creates a graph of several lines instead of the expected single, continuous profile line.
When profiling a PolylineZ feature theme, the profiler will create a profile line for each selected PolylineZ, or for all features if none is selected. In the case of 3D graphics, the tool will profile each selected graphic separately, or it will be inactive if none is selected.
Be wary of lines that come close but don't actually connect. If the PolylineZ is a single feature but not truly contiguous, the profile line in the graph will have gaps corresponding to the gaps in the PolylineZ.