I heard the structure of Shri Sai Satcharita is very specific, but also that some parts of it were lost. Can you explain this?

Yes, the Shri Sai Satcharita has a unique structure. The Preface, discussed in Chapter 0, explains that each of the first 51 chapters is organized with Vedanta first, followed by the Guru's glory, and then a story. The author, Annasaheb, originally intended to include a summary (Simhavalokan) and conclusion (Avataranika) in the 52nd chapter. However, these parts were never found. As detailed in the Preface, Annasaheb had a habit of writing on slips of paper, and it is believed the slips containing the summary and conclusion were lost. Because of this, a new concluding chapter was later prepared and added as the 53rd chapter to complete the work.