Have you checked the source code to the app to see if the values are hard coded.

I found in the past when I manually changed the brightness values in xterm they would then change by themselves after the screen timed out or was locked, I don't know what was controling the brightness, the app or the OS as I didn't uninstall it.

I agree the automatic brightness setting is annoying, but I am now used to it.

As you have disabled the light sensor it would be interesting to see if the brightness gets automatically changed when the screen times out or you lock the device.