Line trace inconsistently passing through landscape on flat component

I have a landscape that is flat on every component except one. Line traces randomly pass through the flat components (depending on where exactly I’m line tracing through them), but are blocked completely by the non-flat component. Here is a picture to illustrate what I mean:

The collision thickness of the landscape was previously set to 16, but after changing it to 100, the problem remains.

What can I do to maintain a flat landscape component, that also blocks line traces and collision checks properly?