[roger64]

Thanks for your replies. I'll have a look. I did not say it was a Kindle "bug". What made it not so "obvious" is that my version of Koreader stayed the same while the Kindle version changed from 5.6.5. to 5.7.2.(or 3)...

Edit: I had a look: indeed Niluje found a workaround and published it for Koreader users.

So what happened could be summarized this way: when issuing its new version 5.7.2, Amazon modified some setting which created a display problem for Koreader. As Amazon does not offer any customization for this particular setting, a Koreader developer found a workaround.

So, what seems "quite obvious" is that there is no bug on either side, just an adaptation to a new rigid setting.