Well, nobody answered... so I guess I'll answer myself.
The text becomes fuzzy/pixelated/grainy after a few page turns. With a screen refresh, it goes away.
I've read this before on this board, some said it was normal, others didn't. So, is it normal or not?
Sorry, I'd actually meant to reply when I read the initial post, but was distracted by some real world things.
I tested this out a little on my NSTG, the few times I saw any odd text were rare and as you observed a refresh would clear it.
I suspect it may be ghosting causing the issue, it's one of the drawbacks of eink. The old image isn't always totally cleared save for a screen refresh. It shouldn't happen all the time, but when it does pressing the 'n' button should cause enough of a refresh to clear it.