Splines on edges are not complete?

I'm using graphviz to format a graph inside of an application using the neato engine. When drawing the edges in the graphs using the data from ED_spl, the splines do not reach all the way between the two nodes that they connected. Is spl all there is to an edge's path or am I missing something (looking at the edge_t structures I have in gdb shows that there's not much else). I'm using version 2.26.3.

Splines on edges are not

You're missing something. The ED_spl information contains an array (usually, 1) of beziers. Each bezier contains a list of points forming a cubic B-spline, plus two flags and two points. If the edge has no arrowheads, both flags are false and the B-spline goes from the perimeter of the tail node to the perimeter of the head node (asssuming headclip and tailclip are true, which they are by default).
If the edge has an arrowhead, the eflag will be true and the point ep indicates the point on the head node's perimeter where the arrowhead nominally touches. How the space between the last B-spline point and ep is filled depends on the shape and size of the arrowhead, though the size is usually related to the distance between the two points. Similar conventions hold for the sflag and sp fields, and the presence of an arrow at the tail of the edge.
You can either provide some appropriate arrowhead, complete the B-spline to ep, ensure edges have dir=none, so no arrowheads are drawn, or equivalently, set arrowhead and arrowtail to none.