Hello Milos and Jean
Using the LibreOffice_7.x.V2_Template.ott.
Copy and paste unformatted text from the previous version of a chapter into the template to create a new chapter file.
Go though the pasted text and add the heading styles to all headings in the unformatted text.
Go though the chapter again to make the necessary changes to text to match the LO version I am working on (now 7.2).
Sometimes I have to change a heading to match the changes. Simply done by highlighting and typing in the change.
Normally there is no visible change in the appearance of the recently typed text.
Occasionally, the typed text looks slightly different and not correct. Reapplying the heading style usually fixes it.
Update the table of contents index and this is when you see the problem, mainly with Heading 3 style.
The text that has been replaced in a Heading 3 style is in italic format in the table of contents. The text in the style that was not changed is OK.
Solution is to go to the heading the chapter and delete it.
Create a Text Body paragraph.
Retype the heading.
Reapply the heading style.
Update the index for the ToC and the problem is fixed.
I am using LO 7.2 for macOS on a Mac mini that uses macOS Big Sur 11.5.1.
Hope that helps in identifying the problem.
Regards
Peter Schofield
psauthor@gmail.com
Technical Writer, LO Documentation Team