On my PB631 I observe a very mildly annoying behaviour of koreader - If the reader is turned on with "open last book" tagged, and the last book was opened by koreader, then koreader opens without refreshing the screen, giving rise to bad ghosting. Only in this case ghosting ever gets to a level which at all bothers me; and anyway one forced page refresh just restores the rendering as it should.
I've opened a ticket at
https://github.com/koreader/koreader/issues/7164 .
Out of curiosity, any other PB owner observes anything the like? I.e. is it a poor waveform tuning on my specific reader, a problem with 631 (as I recall from work of the past months, low level calls required a set of quirks before they worked reliably on its specific kernel), or a more general problem of KoR on PBs?