Hi Aaron, it's hard to say for sure what might be going on there just looking at a screenshot instead of being able to examine the actual model itself.

Is there any way you can post the 3DM model file? You may need to use some kind of file sharing service if it's pretty large.

Are you 100% sure that your first result is a closed watertight solid? How is it that you are determining that, are you checking the object type in the properties panel?

If the original object is not a closed solid then you would get that kind of result. But also you can get that kind of thing if there was some difficulty in forming a completely closed intersection curve calculation during the boolean. There are various things that can get in the way of that like if some areas are self intersecting or the cutting plane possibly just grazes a single endpoint on a pointy area or things like that. It looks like you have a significantly complex intersection there so it would take some investigation of the actual geometry to see what might be going wrong.

One thing you can try to possibly pinpoint problem areas would be to extrude a plane from your line and then select the plane and your base object and run Construct > Curve > Isect. That will calculate intersection curves between things and then if you look around the curve and see missing areas those would be problem spots.

It's just hard to give you any specific details without being able to examine the model though, there are a lot of possible problems many of which can't really be identified just from a screenshot.

- Michael