I suggest it may be similar to the issue i experience when the header displayed is actually for the next header.
To make the report cleaner i ensure that in the group expert, group fields click in desired group then the settings icon and untick the "repeat group header on each page" box.

There is quite a bit of comment on the Dataflex Reports newsgroup about a possible bug which may have had a fix attempted.

It appears to me that the bug still exists.

To reproduce, tick the above box and insert enough lines to either have no detail lines before the new page and then one line on the first page.