I was being lazy and was completing two Tasks simultaneously (3 Beasts and 3 Reds). The 4th slot doesn’t usually matter when farming Challenges, just fill it with parameter Troops (Troops that are for Tasks).
I think you’re on a lucky streak. I don’t track as closely as you do, but I’m pretty sure I haven’t gotten an arcane in battle in over a week. I had a really good run where I picked up 4 or 5 in a couple days before that though.
I don’t know why Talia has this “I’m a troublemaker” attitude - I’ve always found his/her comments to be very useful & clear. Worth paying attention to, certainly.