I don’t know where you are getting your data from, Here’s Sevastopol:
https://www.windy.com/44.600/33.520?42.602,35.684,5
30s then. Still not enough for frostbite. I’m not buying the idea that the Russian Army isn’t prepared for winter weather. If they were Germans or French then I would buy it.
Sevastopol is on a tip of land jutting out into the sea.
In summer, one would expect temperatures to be lower in Sevastopol than most of inland Ukraine, and in the winter, temperatures would be warmer in Sevatopol.