Corrupted Essences not working in maps?

Hi, I've farmed hundreds of essences in maps (T15), and corrupted all of them. I have farmed about 60 greater essences and hundreds of regular essences. I have seen ZERO (0) corrupted essences.

I've heard anecdotes that these essences can not appear in maps, but do appear in the unique essence map only. Please investigate and fix :)