This keeps happening regularly with 4.3.0, too. It starts to look more and more like an OOM situation… I disabled the zram, created a 1GB swap on my home partition (not good for the flash, I know) and raised swappiness from the default 25 to 50. Two days of intentionally heavy application usage all around, and the fingerprints still work. Apps don’t get killed (grayed out in the app switcher) anymore either.