Cenex Wintermaster is a Cenex proprietary fuel that is blended at the terminal and guaranteed to -30F.  Wintermaster and winter blend are not the same thing since anyone can blend #1 & #2 to make a winter blend, only the terminal can blend Wintermaster for Cenex.

I would be pissed if I was OP and was paying a premium for Wintermaster and the supplier was delivering winter blend since Wintermaster.  Wintermaster is far superior to merely blending #1 & #2 since you get the cold weather -30F operating point like #1 with fuel economy like #2.  I will have to agree with the OP if the barrel was empty when filled with Wintermaster this fall there should be no reason that the fuel is gelling at 0F.

https://www.cenex.com/-/media/cenex/files/fuels/data%20sheets%20and%20brochures/cenexwintermaster.ashx?mlts=20200528-192228