I had a Kwikset 912 that eventually not only fell off the network every 1 to 1.5 hours, it would also drive everything else on the Z-Wave mesh off too. A week ago, I finally removed that lock from my HA setup after 1.5 years or so of fighting with it - even changing the Z-Wave module at one point. In another thread, another user found that he, like me, was getting a yellow light next to the ‘A’ button on his 912 lock (my 910). After speaking with Kwikset Support, he was informed that the yellow light indicates a potential of a bad motherboard. Causing a cascade failure with the rest of the Z-Wave devices on our SmartThings mesh was more prevalent once the yellow light was present - probably because the failing motherboard was causing the lock to flood the Z-Wave mesh with junk. In my case at least, all of this is not a recent thing and it is therefore unlikely to have been caused by a recent ST Hub firmware update. As I said, my lock has been steadily degrading for about 1.5 years.
Since changing the lock out a week ago today for a Schlage FE599, the lock - in the same, unchanged environment - with both locks being Z-Wave (not Z-Wave Plus), has been rock sold without any drops by it or anything else on the mesh. The 912 was dropping every 1 to 1.5 hours when I yanked it out.
Other discussions that might shed light one this one: