Index puts page number on new line

I created an two-column index for a document that looked great as shown in first image below. When I went back to the document and updated the index, the format changed and was really ugly, shown in second image below (with formatting marks). I have not been able to recreate it with any version of LIbreOffice, on Windows, or on a Mac.

I deleted the most of the content of the large document and replaced it with some content pasted from elsewhere with index entries inserted. I updated the index and it looks not bad, but is using the same index settings as the one that did not look good (as shown below). I am attaching that two page document.

What I had and want again

sample file

(encoding adjusted by ajlittoz to allow access to attached file)

Your screenshots don’t help to diagnose the issue. In addition you didn’t even enable View>Formatting Marks.

You seem to have formatted your index multi-column. You may have an inconsistent setting of tab stops in the Index n paragraph styles.

For correct diagnostic, edit your question to attach a 1- or 2-page sample file. You do that by editing your question and using the paperclip tool (not available in comments). Don’t use “Add Answer” which is reserved for solutions on this site.

Your sample file displays correctly here (LO under Fedora Linux 34). The only point you could improve is to define tab stop position for the index entries. Presently, they are not defined (equal to 0cm, which is the same). As a consequence, a tab jumps to the next evenly spaced default tab (every 1.25cm = 1/2") which causes page number misalignment. You could instead choose Align right.

In your updated image (with View>Formatting Marks), I see extra paragraph breaks. These cannot be generated by index collection. They must have been added by “some automated process” (because I doubt you manually added them everywhere).

Side remark: most of the sample file is style Default Paragraph Style which should not be used (“common” text should be styled Text Body). Your Text Body paragraphs have received direct formatting. Use character styles instead. Consistent use of paragraph and character styles really facilitate author work.

Your sample displays correctly in my LO on Win 10, even when I change page size to A5 or double-column the page. The colon showing between the title and the name, in your second screenshot, should be a paragraph return. This seems to be an interpretation problem within your installation of LibreOffice.
Does your sample look OK in Safe Mode? Help > Restart in safe mode > Continue in safe mode

The index looks fine in Writer on Ubuntu Linux. I’d not insert a tab between items and page numbers, but instead two spaces or something like that.