Ok, got it now
Indeed there doesn’t seem to be a way to disable the auto-rotation in the stock reader; interestingly I did a test with my Scribe and for some reason my accelerometer seems to be calibrated very differently than yours, as in order to trigger the rotation I really have to flip the device around; 90 degrees is nowhere near the “tipping point”. That’s probably the reason why I never even realized there was no option to turn it off.
It
is possible to do so in KOReader though, in case you need some motivation to make the switch
