Problem:

A document created by writer, format page with 'Area' -> 'Bitmap' selecting
'paper texture', them 'Apply' and 'OK' then 'save as' and file type .docx works
fine, but when the file is reopened by libreoffice, the area bitmap is missing.

The same procedure, saving as .odt and reloading works fine. The bitmap is
displayed as expected.

System:

LibreOffice 6.2.3.2 (Build ID: 20(Build:2)
under Tumbleweed 20190420
KDE: 5.15.4
KDE Frameworks: 5.57.0
QT: 5.12.2
Kernel: 5.0.8-1-default
OS Type: 64-bit

Reproduce by:
1. Create new writer document.
2. Add area bitmap to document
3. Save document as type .docx
4. Close document
5. Re-open document
6. Area background is missing.