I now check every day to make sure my default handling time has not been changed.
I did not receive the recent email referenced by @Uncle_Leroy, however, Amazon did switch me to automated handling time a number of weeks ago. I turned it off as son as I could. My default then reverted back to what it had been before the switcheroo, and it has not changed since then.
We have added our handling time to most of our SKUs, and none of them have – so far – been overridden. Amazon continues to state that it will not toy with SKU-level handling times, but nothing Amazon says or does surprises me anymore.