Your description is exactly matching the behavior of my Kobo Glo (I also stated that already inside the "Kobo bug thread").
So far I always had been pleased with my reader, but this is really not a good job by Kobo
The solution for me was (and is) Firmware 4.3.9084, that was the last one without these issues. Sleep Mode/Light off, Auto Power Off, Battery Life - everything ok again. By the way, of course it brought back the old homescreen...
Fortunately I don't sync the device with Kobo - that would immediately bring the newer firmware again
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