Generally speaking, Word expects to create one copy of your letter for each
record in the datasource. In other words, Word mailmerge is not designed to
stuff a set of records into the middle of a document, i.e. does not really
work the way you are hoping it will.

So, what can be done? It depends on precisely what you're trying to achieve,
but the simplest way to achieve what you want could be:
a. create a completely new mailmerge, using a "catalog mailmerge". This
would have a single table row with one column for each column you want to
merge from the datasource. Attach the mail merge main document to the data
source, and insert a MERGEFIELD field in each column of the table row.
b. perform the merge, outputting to a new document. You should have a new
table consisting of the rows you want.
c. copy/paste that table into your letter.
d. create a header row by selecting row 1 and copying/pasting it, then
replacing the copied data by the table column headers you want.

There are other possibilities...

Peter Jamieson