This is because the firmware itself does not carry this font.

For Kindle, the temporary failure of these two fonts is a "normal phenomenon". This may be because the firmware itself does not carry the two fonts "Yuanti" and "Kaili". They will be erased when updating the firmware, so you need to pull these two fonts from the cloud again.

The Amazon Kindle is an e-reader (and software platform) designed and sold by Amazon. The first-generation Kindle was released on November 19, 2007. Users can use Amazon Kindle to purchase, download and read e-books, newspapers, magazines, blogs and other electronic media through wireless networks.