Yes it seems to have corrected the problem. I say seems because I have had a couple of drops since the change but I know one was due to 802.1x MAB and I suspect the other was due to something different. I check all ATA most Tuesdays and Thursdays and have not seen drops like I did before. So I say it fixed my problem.
I am seeing the same issue (ATA going unregistered after an extended period of time, 1 or 2 months) using CUCM System version: 188.8.131.5200-115. There are a total of 135 ATA 191 units in this environment. During the last two months, over 70 units have become unregistered from CUCM. line-2 support has been disabled on all units and issue persists. No other VIOP devices, even on the same subnets as these ATA 191s, are affected. I have opened a third TAC case regarding this issue yesterday (Issue was never resolved). I have setup a controlled testbed with 14 ATA units and am prepared to perform packet capture and continue troubleshooting. Please share if resolution is discovered, I will do the same.