Medicated feed is a thiamine blocker amprolium most of the time, and from my limited knowledge, it blocks thiamine by being a thiamine analog. In other words, a slightly modified copy of thamine, that binds to thamine receptors so the real stuff can't do it's work. (Inhibits cocci from reproducing, so helps build immunity, does not work like a vaccine.) So it would likely be even more stable at freezer temps, even though it is just fine at room temp.
As for storing in the freezer... I just store up to about 300 lbs in the hallway of the house for up to about 3 months just fine. We eat rice stored all year so figure the feed for a few months is fine. Grains can be sprouted decades after being stored, so the nutrients are somewhat still there, granted, they are ground up in the case of feed.
Oddly enough... I've never had an issue with bugs in feed in over a decade? Either that or I am blind.