I started writing a book in Google Docs, making heavy use of headings to build a table of contents. At 150 pages, G docs was too slow, so I switched to Libre Office. Initially I used .odt, then switched to .docx around page 200,
So I did the majority in .docx get up to page 260 and in November the year before I had enabled “track changes” so I could have a better chance to recover from stuffups, Recently for backup purpose and Save as of the doc and chose “Accept All Changes” for the new docx going forward, keeping a frozen copy that i change the name of but don’t work on to preserve the tracked changes. At 65,000 words, 260 pages. and 74.8 MB I wanted to see if it would get smaller when I accepted all changes. Yep it went to 74.1 MB.
Shortly after doing that Libre stopped being able to open the file at all! crashing on open every time Libre v7.3.7.2 (on linux mint). Luckily I was able to boot into MX Linux on a spare partiton, which had Libre Writer 6 I think - it was able to open the half dead .docx file! So I saved out two copies: Book-RESCUED.docx and Book-RESCUED.odt
From here I think I willl finished the book off using ODT format! If you think about it, makes sense to use the native format.