Problem using Navigator after downloading a Google doc

I’m trying to move away from Google Docs and have downloaded them all as both .docx and .odt in order to get around this problem, and it’s the same either way;

When I open the downloaded document in Libre Writer, and open the Navigator, it shows no headings under “Headings”, but under “Bookmarks” it does show is a list of bookmarks with scrambled names like “_89zmvlfha8”- these actually jump to the different headings when I double click them, so something has gone screwy in the conversion I guess.

What’s crazy is I can’t even fix it by clearing the formatting of a heading and re-applying the Heading style. It seems the styles are permanently borked for this file. The only way I’ve been able to fix this is manually create the document again by pasting as plain text in a new file and re-applying the heading styles. If I don’t paste as plain text, the problem carries over to the new document! Any ideas?

I’ve made an example file available here;

Whatever has generated that file, it didn’t set the outline level of Heading N styles in a usual way: they all are set to “Text Body” - compare the Outline & Numbering tabs of the styles’ properties in the document and in a new document. Outline level is what is shown as Headings in Navigator.

Ah, life saver, thank you! This is easily fixed per document. Wow this is a pretty bad oversight on Google’s part.