That's a downside of using a now unsupported LaTeX module (tabu) - I'm not aware of a ready fix for this. Migrating to the new shiny table module is on the todo list but there's downsides other than the amount of labour required.
It only affects a couple of tables badly and whether it is visible at all depends on the PDF reader implementation you use. For instance, I can't see it in browser-based PDF readers.