Maybe use Amazons way of fixing rendering issues on some files (just in case) and see if it helps:
This is from Kindle support after asking about a book that does not render right (font spacing problem) adjust instruction to your favourite way of loading book to Kindle.
Quote:
Hello,
I'm sorry you've had problems viewing [enter boo ] on your Kindle. Sometimes deleting the title from your device and downloading it again will resolve the problem. Here’s how:
Before you begin, turn Annotations Backup on to save any notes you made in your item:
1. From the Home screen, select Menu and select Settings.
2. Navigate to Annotations Backup and select turn on.
Step 1: Delete the item from your device
1. From the Home screen, tap and hold the item.
2. Tap Remove from Device.
Step 2: Restart your Kindle
1. From the Home screen, select Menu, then Settings.
2. Select Menu again, and then select Restart.
When your Kindle restarts, go to Archived Items (or the Cloud tab on Kindle Paperwhite) and tap the item to try and download it again.
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good luck, can't really help any more - other than you maybe trying to create a epub, and then converting to mobi with calibre or kindlegen.