I keep feed in the original bags inside plastic trash cans with lids. If I had a place to store the cans out of the sun I would, so I do the second best thing, which is store them where they are in shade in the afternoon when sun is hottest. I only buy what I use in two weeks or less.
It's just the one, not spreading/multiplying, not on any other birds? I don't know what it is, just asking because I think if this is all you are seeing I think it's more likely to be just an injury.
Seconding @Wyorp Rock here - we've all had things go wrong when we didn't expect it, and sometimes struggled to figure out what was wrong and how to correct it, if it was correctable. I still have things go wrong. Bouncing it off us is just a way to brainstorm possibilities.
One thing to do if...
Do the feed and corn have any odor other than normal? Any sign of parasites? Any lawn/garden treatment with poisons that could have drifted over? If nothing has really changed, then I'd suspect either feed or parasites. You can get a fresh bag of feed and no corn and feed only the fresh feed...