I use the same weapon but I curse manually or with Aura of Frailty and do not have Grim Scythe in use.
I believe that curses from the weapon do not behave like real curses you (auto)cast yourself.
Grim Scythe (skill)
I use
IActor.GetAttributeValue to query if curse is applied to each monster (actor). you have to know exact arguments for the function to work. Specifically the modifier.
I found them for these three curses somewhere in this forum and accidentally they are same as:
Necromancer_Passive_FrailtyPassiveEffect
Necromancer_Passive_DecrepifyPassiveEffect
Necromancer_Passive_LeechPassiveEffect
Maybe somebody with better knowledge can tell what attribute modifiers to use in
GetAttributeValue to get effects (like curses) from Tragouls Corroded Fang when it randomly "curses" monsters.
One option to debug this further is to use Ctrl-Alt-D and then open "dump_actors_on_screen.txt" in logs folder and try to find correct attribute modifier values from there.
I believe that -1 after attribute name is that modifier "position".