Just a hunch: When columns are not perfectly aligned, Word tables can get confused. This can happen, for instance, when a cell's vertical border is moved, inadvertently or not.


It can also be difficult to see. I recommend enlarging the view so that even a slight misalignment will show. Another detective technique is to select each column individually and see if the selection jumps to a horizontally neighboring cell at any point.

If you find a cell wall that needs adjusting, select the cell, with or without adjacent cells, or the row, and use Alt+drag to fine tune the alignment at an enlarged view.

Hope this helps.