The only other thing I wonder...quite a while back I think I remember discussion where a "prc" book wasn't really a "mobi" book.
...I went off looking to see if I could find what I thought I remembered...
There are some "prc" files that if you open the file in something like notepad there is a string you will be able to see in the first line. A proper "mobi" prc file you should see "BOOKMOBI", but some prc files will show "TEXTREAD".
Try this thread to see if there is anything relevant?
https://www.mobileread.com/forums/sho...t=prc+textread