SSRS: Keep together ignored when generating von docx reports

Hello,

we have problems creating docx documents with SSRS.

The main report contains several subreports.
One of the subreports contains of a tablix with some textboxes (as headlines) and another subreport for the details.
The inner subreport contains as well one tablix, this one with a rectangle to keep the details of one datarow together (placed in several textboxes). In the properties of the rectangle we set "kepp together" true.

Genarating a PDF works as expected.

When rendering as a docx, we have a page break between the headline from subreport1 and the first rectangle of subreport2.
Ther are some page breaks in a multiline textbos and between textboxes inside the rectangle.

Following is an abstract of the rport structure:

MainReport => body => subreport1
subreport1 => body => tablix1 => datarow of textboxes & subreport2
subreport2 => body => tablix2 => rectangle (keep together = true) => datarow of textboxes

Now my question: How can i afford, that word (docx) treats the "keep together" property of the rectangle as desired
and how can i avoid the first unnecessary page break before the first datarow of subreport2?

Thanks.