Page Numbers depend on a file called {book}.apnx, which calibre can auto-generate for e-ink Kindles, but not for Kindle Fires. Kindle Fire is a whole different ballgame in a bunch of ways, for a bunch of things.
This also can only work when sideloading by connecting to calibre as a device and using Send-to-device. When using the Content Server, you cannot receive extra files like that.
Also, the Page Numbers are a guesstimate.
Bottom line is, when trying to hack the device to use special features not deliberately made accessible, one must not expect any guarantees.