ToME: the Tales of Maj'Eyal

I had it a number of times during my current playthrough that I saw a monster, but when I moved my mouse on top of it to see the info, the info window flashed an then the monster and it's info went invisible, as if it entered stealth. I think I killed all of these afterwards with my alchemist bombs but I am not 100% sure.

Yes its annoying, but not a bug.

Well, either it should be visible or not, disappering when I hover above it I would consider a bug in a turn-based game.