A family of Microsoft word processing software products for creating web, email, and print documents.
A while ago there was a post regarding an interaction between table cells that were vertically-centered or bottom-aligned, and graphic objects placed above it (particularly if they were in a header). In that case, the graphics that intruded on the table caused part of text in the last cell to disappear (that would be exactly the opposite of your problem). The solution was to top-align the cells or change the top object's position.
If that kind of alignment is not your case, consider replacing the lines by cell borders (use the Tables tab in the Ribbon). It will be better if your table is not floating (Table > Table properties > None), so it cannot occlude the floating lines.

