I'm trying to add hatching to a drawing. I'm having an issue where the boundaries of the areas to be hatched are a mixture of straight lines and curves - drawn as splines. 

In a limited number of situations, the hatching has worked in this drawing, and it worked well on other drawings. However, when I try to create many of the hatches I need, Draftsight correctly recognises the boundaries I want to use, but when it creates the hatch it hatches a shape defined by straight lines between points that define the intersection of the bounding entities - specifically where the splines intersect with other entities. And the spline boundaries are ignored. In some cases, no hatch seems to be created, but there is no error message either - it just exits as if everything went smoothly. 

I've tested it with other boundary types, lines, polylines, arcs, and they all work fine. If I create a closed spline shape - no issue. Only an open spline with some other entity type causes this problem.

I've confirmed all the entities I'm using are flat and in the same plane (z=0). I've confirmed the hatch is not going to a hidden or frozen layer. I've created hatches with both significantly larger and smaller enclosed areas and they work fine (so long as I don't use an spline combined with another entity). All the entities I'm using were created by myself, natively in draftsight, and are on a layer I also added to the drawing. There is no change if I break the bounding entities at the relevant intersections or not. 

I've attached a short screen recording showing the behaviour on a simple area (zoomed in to maintain the confidentiality of clients drawing).

I'm at my wits-end trying to solve this problem. Does anybody have any hints as to what could be causing this and how I could resolve it?