It seems like poison damage is both causing a gradual damage decrease and still doing a “tick.” Here is a slowmo video demonstrating the effect. Also, it’s not the hungering souls doing the big hits as those barely do anything and there are two ticks despite the souls only hitting once.
Compared to earlier patches, this makes the damage seem way higher, so I think that poison might be dealing double damage by doing a gradual and tick decrease of health!
EDIT: It seems the issue might just be Acolyte poison related? People have told me their AoD also deals excessive damage, and I agree the tick damage for 1 second worth of poison in a level 100 monolith seems very high.