When I open an “xlsx” file I often see the border line become extra bold. When I make the line it was a very thin (0.5pt or any number below 1pt), but when opened, it became 4.0pt.
Try if changing the option in:
Menu/Tools/Options/LibreOffice/View/ Use Anti-Aliasing
shows the lines better.
I have the same problem (version 188.8.131.52 MacOS 10.8.3). Unchecking Use Anti-Aliasing did not make any difference. This is a real deal-breaker for me; I may have to switch to a different office program if this is not fixed soon.