The Onyx firmware has the nice feature of switching to A2/fast mode in some Views as text is being scrolled (e.g. WebView), then returning to Greyscale Update mode as the scroll is finished.
At least one user (user “thine”, see https://www.mobileread.com/forums/showthread.php?t=293789&page=35) has reported that one application (the “The Guardian” app, “com.guardian”) does not revert to GU after having switched to fast mode.
Maybe, to prevent the problem of exceptions such as this one, an option could be added in the display settings to disable automatic switch to A2/fast mode?