Currently using a team with my S5 3-4* to use the Crook and Flail.
I am also using Shen and Crystal Benben Amulets.
Full team is Ahhotep / Ptolemy / Junaid / Azmia / Rekhetre
When Ahhotep fires, his target gets the buff rather than him getting the buff. It seems that the game is considering his buff to be a debuff, so it reflects from the Shen amulet, but I’m not sure.
Ahhotep applies a debuff to the enemy like a “mark”.
If that enemy is killed with the debuff still active, then it should proc and give the 20% mana to the caster’s allies. If this is not happening, can you maybe screen capture the process, or record a short video clip of it occuring?