I also use KoReader, you'll notice that if you hold the device upside down the buttons don't change. You'll need a patch for that, it's very simple though.
Patch thread:
https://github.com/koreader/koreader/issues/9223
Save the content below in a textfile, rename it to
2-disable-hw-button-rotation.lua and place it under
/koreader/patches/
Code:
local logger = require("logger")
local Device = require("device")
Device.input:disableRotationMap()
logger.info("Hardware button rotation disabled by user patch")