The "Digitize Network" tool can be used to draw new pipes and junctions, but does not split existing pipes, even though you can snap to existing pipes and add a junction. This can cause confusion because it appears that the pipe has been split when it actually has not.
The functionality of splitting an existing pipe exists when using Edit-Insert-Junction. Makes sense that the Digitize Network tool should behave similarly when last length of new pipe is snapped to an existing pipe.