Description:
When saving an Impress document in Microsoft PowerPoint 2019 format, the text highlighting is not saved.

Steps to Reproduce:
1. I create a new Impress document. 
2. I type "hello world".
3. I select on word, for example "hello" , 
4. On the Formatting bar, I click the Highlighting icon.
5. The word hello is highlighted yellow (correct).
6. I save the document in Microsoft PowerPoint 2019 format. 
7. I close LibreOffice. 
8. I re-open the document in Libre-Office. 
9. The yellow has disappeared. (not correct)



Actual Results:
9. The yellow has disappeared. (not correct)

Expected Results:
When reopenning the document, the highlighted text should be yellow. 


Reproducible: Always


User Profile Reset: No



Additional Info:
If I open the document in Microsoft Office, same result, the highlighting is not there.