In a document which is numbered by chapter, the index entries reflect only the page number within a chapter. Entries appear as 2,5,5,7,2 instead of 3-2,1-5,4-5,2-7,3-2.
The root of this problem is discussed in Libreoffice bug 105628. The workaround is similar to that of a previous question ( Chapter number not in all TOC levels ). For indexes, the fix is
- Edit the index
- Add the Chapter Info field (also referred to as CI)
- While the CI field is selected, make the evaluate up to level a 1.