Despite the misleading name ("Enhanced") I don't think development of the deliverable named "XMetaL Enhanced PDF via RenderX XEP" has kept up with that of the rest of the DITA Open Toolkit, so it will have limitations like this. This deliverable was originally included to overcome a large number of limitations in the early versions of the DITA Open Toolkit deliverables (adding support for Bookmaps, indexes, sorting, various elements, etc. that the DITA OT versions of PDF output did not handle), however, it has since fallen behind as development on the DITA OT versions of PDF has progressed.
If you use one of the other PDF deliverables they are likely to give you what you want and are being actively developed (Book via RenderX for example).
Thanks for the help. Yes, the output is as expected when using "Book via RenderX". So, once I found the source files for this output (org.dita.pdf2), I compared the implementation with my default implementation. I discovered that, by default, there are no table borders (obvious now I know). In our default implementation, not sure if this was done by us in the past, but there was some code that put borders in even with colsep or rowsep set to 0, so I simply commented these out and it now works fine.