Old thread, but wanted to see if anyone else was having the same issue since I'm going back at this achievement again. Yes, this is a very common issue. By the time I had 100 Trove Scamps, I had roughly mid 30's of Cunning Scamps. Looks like you're on the same route. The past two days I've been solo farming (best done with 2 people because of their spawning pattern), and I've only had a few Cunning. Troves? Plenty of them (though, as I write this, I've gotten 3 Cunning in a row, so that's probably my Cunning for the month, knowing RNG, lol).

Why ZOS decided it was a good idea to require you kill 100 of EACH when one is clearly more rare than the other, I have no idea. Most Slayer achievements in dungeons, for example, are properly proportionate to where you'll be required to kill a few hundred of a common enemy, but roughly 40-50 of a more rare enemy in the dungeon. This is the case for places like City of Ash 2 (Veteran Flame Colossus Slayer; 50) and Icereach (Frost Atronach Slayer; 45). If it was 1:1, or if one area spawned Trove and another area spawned Cunning, that'd be much better since players could have the opportunity to farm which ever they wanted while still respecting the waiting game on the 5 minute spawn timers, but instead we face RNG.

I'm currently at 64 Cunning. 1 more year and I may have this. Hope your Scamp adventures have yielded better results!