Actually Sif is designed to counter with the full incoming damage then to cut the received in half, which is why she only does 100% as opposed to the other countering heroes. Is that not how it has been working for you?
Sif is working great for me, I haven’t been booted out of Diamond since maxing her out and running her as my Tank, I couldn’t stay in Diamond before that as I have no “True” 5* tanks. I use her quite a bit and she, and the flanks, definitely only take 50% of the damage while the special is active
I don’t have Sif, but if ever I got her, I rather have her skills as it is. I think it is a better riposte than Botil, Cyprian, Elena and Sumitumo since she only gets half of the damage dealt by the enemy but they suffer the whole brunt of the damage they are supposed to give. Her riposte makes her survive longer than Boril, Cyprian, Elena and Sumitumo as they receive the full damage dealt by the enemy.
@TemixCO7 - I am not sure why you’re facing this problem. Please provide the screenshots as @zephyr1 suggested.
Sif is an integral part of my Yellow stack… I just raided with that stack a few minutes before opening this thread. (Sir Roostley, KillHare both fired and killed themselves… while Sif and her adjacent mates stayed alive. ) Anyways, no bug as far as my experience is concerned.