It depends how fast you’re playing. The average drop rate for Arcanes in explore is around 6% (so one Arcane every 16-17 battles) with Celestials about 3%. If you’re playing around 15-20 battles per hour, that would be reasonable for Arcanes and good for Celestials, if you’re playing faster than that, I’d say you had a bit of bad luck.
If you did that with a fast bombot team (I average less than a minute per battle), that would be a pretty disappointing session, though I have experienced similar results myself, only to get 4 Arcanes in 15 battles at other times.
All that to say, I haven’t seen any evidence of a secret nerf to Explore, though I haven’t done much myself this week.