Use it a lot on K3
Have a K3 and use Fetch annotations a lot.
Keep in mind that the Kindle is responsible for writing data to the mbp files with correct reading progress. This can fail ! IF your kindle clock is out of sync this could happen. Also some books might have issues.
However here's what my rutine is:
Read, read, read and get to the end of the book.
THEN I put K3 to sleep using power button, to ensure reading status is updated.
Wake up K3, use the back button to get back to what menu I want.
IF there are problems with reading status, you'll see it right away as the back button will NOT exit the book, but jump further towards the start of the book.
I DID have a few books that couldn't handle the rutine above.
A dirty hack (not recommended) was to open the book again, fast turn pages until the end, then press Home, Menu, Settings, Restart, and for some reason the mbp file was update correctly.
As Kovid states different Kindle models with different firmware versions can have a huge impact. A shame the function is not maintained, but so far I would say that in 99,9 % of my many books it works like a charm.