If your bulk tank is freezing it’s set too cold and you stand a chance of developing rancidity in the milk. This is because the formed ice crystals puncture the fat globules, releasing lipase. Be sure to not switch the tank on too early - some people like to do this to give the tank a “running start”, before the milk is pumped in.
When the milk freezes, it also concentrates the milk solids, making the milk sweet and increasing the viscosity.