Updating index creates unpredictable formatting

I have created an index in Libre Office Writer by manually selecting an entry and adding it to the Entries box. The initial result was fine. Upon saving, closing, reopening the file and updating the index I have noted that all the entries have migrated to the 1st key box and have gained a colon. The index now places the page numbers on a separate line from the entry. It has also created a problem for book formatters upon exporting to pdf. I do not understand this problem. I would like to fix it without hand changing each entry. I would also like for this to not happen again. I am on Windows 10, Version: (x64), saving in docx format.

(Edit: activated screenshot -AK)

Edit your question to attach a sample file (use the paperclip tool when editing).

Mention your OS name, LO version and the format you save the file into. Note that a format other than .odt bumps into compatibility issues.

The quick fix is to keep working in native format ODF (.odt for text). Then export to PDF when done.

If you need to provide documents in .docx with such not that simple features, then stick to MS Word. This is the vendor lock-in policy by MS.

See here for some details: [Tutorial] Differences between Microsoft and AOO/LO files.

It is also worth mentioning that there will be “formatting residue” buildup from converting back and forth between Writer and Word (with workarounds inserted for each “source” feature not available in the “target” data format). This may cause considerable problems. Conversion happens every time you open your file, and every time you save it.

At worst you may need to recreate your document from a plaintext copy, redoing all heading and index entries, figures, illustrations and tables, etc. Hopefully not required, but if you see erratic behavior also after you start working in native format, it may be time to “cut to the bone”.