If you right click and split the route it behaves correctly.

If you left click to create a waypoint and then left click again to split the route it behaves correctly.

But: If you left click and drag the new waypoint to change the route slightly and then immediately left click again and split the route the distance shown (in the left hand panel) will be nearly doubled. And, if you follow along the path (via the elevation profile and watching the small circle) you see that it goes along the path and then at the end goes back to the start in a straight line and then starts along the path again for a little bit.

If you left click and drag the new waypoint to change the route slightly, drag it again to adjust, and then left click to split the route it behaves correctly.