It's closely related to the device being in a low power state (read: standby) at the time the buttons are pressed, meaning said input events become the wakeup source.
The hardware and/or kernel appears to sometimes lose track of the actual input events, and never delivers them to userland.
At the cost of severely gimping your battery life, you can disable autostandby in KOReader (or simply not enable it in the first place, as it's opt-in
), and that'll effectively workaround (in a very literal sense ;p) the issue
.