Good reading and excellent information.
I will admit that snow tires will handle VASTLY different than all-seasons in snow.
IF the all-seasons were to blame for the rear end trying to slide around me, why didn't I experience it from the get go, with tires on a vehicle that had crappy suspension, aftermarket drive axles that seem to be suspect for a number of faults, and was last aligned who know when?
IF the all-seasons were to blame, then why did the car literally drive/behave as expected one day and was an entirely different beast the next?
It's almost as if it's giving more power to the rear wheels and less to the front. Honestly, I wondered if the front was getting ANY power, so I popped the fuse in to disable the rear drive and drove to the end of the street and back on the front drive alone. It handled exactly like a front-wheel drive, but with no sliding like I had just experienced.
I did find some NWS data for comparison, and I can't see any significant temperature variations from the winter of 2017/2018 and this year 2018/2019 (or even just this year) that would make me say "A-ha! THAT'S why!"
As for "All Subaru 4x and AWD have had a slight rear bias from the beginning of time", then why didn't I experience the sliding rear in December with a Forrester on all-seasons? (Rental in Virginia, 7 inches of sudden, unexpected snow.)
I grew up driving nothing but front wheel drive vehicles (and none ever had snow tires, btw). As a result, I have developed a healthy respect for winter driving: Anticipate slick conditions, don't drive too fast, don't make any sudden moves, etc. (Ok, I do admit to using the hand brake more than a few times to slide around my corners.) These are the habits I carried over to driving the Subaru. These were the habits I was exercising when the Subaru started acting so suddenly squirrelly.
Maybe... My incredible never-had-any-issues streak *was* the anomaly and This is what normal actually is?
Or it was...… Everything aligned just right to have it slide around on you this one time.
Did it do it just the once, or has it done it more since? Perhaps it's just an anomaly?