Oh, of course. The extended driver makes them into kepubs, which allows you to use the statistics feature, I remember that now.
Well, apparently it also has these side effects.
Here's a photo of the same book on two devices (a free epub from Feedbooks):
https://fmlekens.home.xs4all.nl/nonc...riversDiff.jpg
On the one full justify can effectively disabled, on the other it can't.
Wish I had thought of that before, sigh.
(Although sideloading 1500 books at once doesn't seem to be a good idea with the extended driver. So there's that.)