I'm not sure how to reproduce it consistently, but I see that sometimes segment highlighting goes out of sync. For an example see the following screenshots.
Here the mouse pointer is out of the columns, but a segment is highlighted:
Here the mouse pointer points at the segment, but it's not highlighted, while the corresponding source is highlighted:
(You'll have to believe me about the location of the mouse pointer - it's not visible in the screenshot.)
This probably happens because the source and target modules use toggleClass with cx-highlight, but I didn't debug deeply.