I've been stuck on 23/50 for several days now and it hasn't progressed any further and to be honest I don't know if it's bugged or if I'm doing something wrong. What little information there is to find about this is vague and often times different. No matter if I try to avoid traps in the game (mines, skulls, hazard rooms) or if I set them off, it no longer seems to count towards unlocking the DoT skill tree. It was mentioned somewhere that trapped chests count towards this as well that that doesn't seem to be true.