I've never trusted plastic bags for seeds. I get seeds packed like that from Lily Auction sellers, but because of experiences like yours, I don't leave them there.
Plastic is impervious, and contact with plant tissue promotes decomposition. I noticed this when storing swiss chard in the refrigerator. In a loose plastic produce bag it would start to noticeably degrade after one day, but if I wrapped the leaf bundles in paper towel, then put it in the plastic, it stored well for several days.
I started shipping daylilies that way —wrap the roots in paper towel, put them in a plastic bag, dampen the paper lightly, then loosely tie it at the crown—and they always arrive with fresh, moist roots.
It's an added expense, but I've had the best results putting my seeds in Brown Kraft coin envelopes, and grouping those in ZipLoc baggies. I refrigerate them dry or *slightly* damp around October. Two months before I want to plant, I put about an inch of water in the ZipLoc, let it soak the bottoms of the envelopes for about 5-10 minutes, drain off the excess water, then store them *vertically* in the fridge to stratify.
The paper envelopes will eventually get a little moldy, but the seeds and seedlings inside are healthy. There will be a rotter here and there, but I figure those probably weren't viable anyway. There's a lot of air in the ZipLoc, and the environment is so nice that even the rotting squishers don't affect the good seed. Once I used a weak Physan 20 solution for the soak, and the envelopes stayed very clean throughout storage.
Many of the seed will germinate in the envelopes, (which is how I like to plant them) but because of the cold, they develop so slowly that the window for planting is generous. It's actually nice to have a little emerging root to push into the soil, or even a short leaf "handle" to hold them by.
The envelopes I use are Columbian No. 3 Coin Envelopes #CO545
