The smell of my genmaicha changed?

Almost exactly two weeks ago, I got some genmaicha from argo teas. When I first opened it the aroma of the sencha was very strong and I could smell the distinct seaweed-like scent of japanese tea. Just recently however, I’ve realized that the scent has totally changed. The aforementioned seaweed-like smell is almost imperceptible and a somewhat sour/acidic scent is now dominant.

So my question is, is this normal? Maybe the initial scent was something that accumulated over time while it was in the store and was quickly released in the first few openings. Or has something changed in the tea? If the latter, what did I do wrong? I kept it in the glass container it came with, but I usually kept it in a dark place so I don’t think UV light could have affected it so much.