Issue with certain mercenaries not spawning.

I've been searching for a specific mercenary for two weeks, but the mercenary called 'Cruel Mistress' hasn't appeared.

I've run hundreds of maps and encountered the enemy less than 30 times. I want to get a mercenary with a specific skill, but isn't this a bit too much?

It's not just me—several others are experiencing the same issue.

This clearly seems to be a bug. I strongly urge you to look into it and provide a proper explanation.

I have seen several of them - I have a level 83 one, so they do exist.

RNG.