Any ideas?
Not really - the fact that it works witha different type of printer suggests
that either there is something wrong with the Brother driver, or that - and
I don't know how to put this - Word does something out of the ordinary when
it tries to work with the Brother driver. I had a quick look at the
troubleshooting pages on the Brother website and noticed for example that
they mention problems when other printer drivers are also installed etc.
(FWIW, for a long time I had a very nasty problem when I had an Epson and HP
attached to the same machine. Of course they should have worked together but
in reality the only resolution was to disconnect one of them).
Faced with this situation myself, I would have a good look through the
troubleshooting materials on Brother's website, try to isolate the problem
by simplifying as much as possible (remove unused drivers/printers, have a
good look at driver options, try merging to document then printing vs.
printing directly (although I think from what you said you must be merging
to a document anyway) and so on). It's possible that there are also "MS"
(supplied with the operating system) and "Brother" versions of the printer
driver, too. I'd try both. MS Print spooler settings might also be a factor.
But in the end, if you have a Word document that prints correctly on one
printer and not on another, I suppose you have to suspect the
software/hardware package that does not work and contact the hardware
supplier, in this case Brother.