It's all very strange. Magnolia stellata is hardy to -25 C and lower, according to the magnolia specialist sites, and it grows in Zone 4, -28C, in its native Japan. My MIL had one in her garden in Wales, 1000 feet up, where we had night temperatures of -13 C and never above -3 C for two weeks one winter, and it was fine.
Magnolia stellata has moderate drought tolerance when established as yours was, provided it is planted in good moisture holding soil, ideally acidic, though it will do well in alkaline soil in humus rich soil. If your magnolia was suffering from drought then I think you would have noticed it shed quite a lot of its leaves.
I have lost most of my ceanothus, all my established bay trees, and my two chenomeles Crimson and Gold have been cut dead to the base, where there is some new growth. I have never had this before. The chaenomeles is supposed to be as hardy as the magnolia stellata hybrid that has just flowered brilliantly in my alkaline soil - down to -25 C!!
Maybe it is the cold weather - maybe very hardy plants grown in relatively mild climates get 'soft' and then are vulnerable to unusually low temperatures. MIL's stellata was used to much cold winters, so maybe it grew up tough.
I have decided that I am going to replace my ceanothus (Concha) with the same variety, in the same spot, with added Rootgrow. I am taking a risk, but I can't believe we'll have two such hard winters again for quite a few years. Maybe you should do the same.