Words randomly aren’t added to Kindle Paperwhite vocabulary

I have been using my Kindle Paperwhite for 1 year for reading Japanese books. I have added a JMdict dictionary, and the words that I looked up were added to the vocabulary. Late,r I could export these words and study them.

But one month ago, something changed. I wanted to export the vocabulary and saw that the file wasn't updated for 1 month – no new words appeared in it.

I tried deleting the vocabulary file on my Kindle, but somehow it got restored (maybe from Amazon backup).

I tried doing a factory reset, but it didn't help – the vocab with the old words is back, and the new words don't always get added. Sometimes they do, sometimes they don't. It doesn't matter if the word is present in the dictionary or not.

I don't know what to do. Any help?

by Artgor