Hi Abbie. Thank you for the detailed help. 🙂
As you can verify in the attached image, the index file types are added for mobi, azw, fb2, and xps, but Anytxt 1.3.1517 (regular / non-ocr version) is not searching inside those files.
I have ensured that these files do contain text in them which can be searched by opening them in a reader like Sumatra.