Just my thought, but this time sage key drop rates are a lot lowered.

We can get 5 key/day from plot/elite instances. Earlier, whenever sage treasure event was up, we always got 5 sage key from the first 5 sweep we did. This time however, things are a bit different. After doing the daily 60 elite sweep for monthly event, was surprised to see that I had 3 keys only, so had to make an additional 100 sweep to get the rest 2 keys.

Maybe you are having some bad rng luck as well? How many sweeps have you done so far?