I finally learned - the hard way - my Maangchi stores her red pepper flakes in the freezer. This is my sad story.
There was my 1kg bag of pepper flakes in my pantry: a dark, dry place and ideal for storage. I had the flakes double-bagged in Zip-loc bags. No problem, I thought. WRONG.
After a couple of months of not using said flakes, I went to the pantry, and lo and behold: the beautiful red of the peppers had gone and instead I had these pale coral-coloured flakes. I opened the bags, and ugh - they were musty, damp, and clumpy. Completely off. I had to toss it away. Almost a whole 1kg bag! *sob!*
So I opened up my back-up bag, and this time I have done as Maangchi does: put it straight in the freezer!
I'm not sure how or why my flakes went off like this, particularly since dried chilies and chili flakes keep for such a long time, but I'm guessing Korean red pepper flakes are far more delicate and possibly have a higher water content after drying. (It wouldn't take much water, by the way.)
So if you're wondering why you should store your flakes in the freezer, wonder no more, and learn from me!