Well, I went back to 4.34 and the bad news is that the page skipping bug still happens. With both buttons, so I don't think it's the hardware.
I guess the next step would be the factory reset and reinstalling 4.34, but I don't know if I want to go to so much trouble. And who knows if even that would help. Grrr. Damn Kobo and their buggy firmware updates.