Odd problem with table borders in Word 2007

I am a power user in a publications department, and we have come across a bizarre problem that has us stumped.

One of my coworkers created a table on a landscape page to be used for customized notecards. She used a 3pt default groove border (which has both a thick and thin line) and applied it as a box around each of the four main squares, which have spacer cells in between. The border wraps around like a frame on all corners EXCEPT the lower-right corner of the top cell (on the right side) and the upper-right corner of the bottom cell (on the right side). For some reason, it appears Word is attributing the border to the spacer cell in the middle instead of the top and bottom cells - but ONLY on the aforementioned corners - thus treating them like straight lines that don't join to anything. Consequently, there is a little white line in the middle of the groove border that slashes through those corners because they aren't wrapping around.

We've tried changing the page margins, reducing the point size of the border, adding extra columns/rows, manipulating the cell padding of the table, recreating the table, etc. - but nothing will make the inner white line between the thick and thin lines wrap around on those two corners.

I did a bunch of google searches, but apparently this isn't an issue that has come up for anyone else (?) Any thoughts would be appreciated - thanks!