I am not sure if this is your exact issue but a similar issue is caused by a known bug reported in Bug 21835348 - GRID LINES DISAPPEARING IF CUSTOMIZE GRID FORMAT IS ACCESSED WHILE ENTERING GRID.
I am not sure if this is your exact issue but a similar issue is caused by a known bug reported in Bug 21835348 - GRID LINES DISAPPEARING IF CUSTOMIZE GRID FORMAT IS ACCESSED WHILE ENTERING GRID.