Nothing new dude , that also happen in both Demon´s Souls and Dark Souls one. It sucks but has been part of the franchise ever.
I had never been in a situation where I kill the boss and then die right after the "Victory Achieved" message. However, if you see Vinny's stream of Dark Souls he died from Power Within during the fight with Gwyn after Gwyn went down and it still counted as a victory.
@demoskinos Yes, the boss at No Man's Warf. I've killed him plenty of times when assisting and I can dodge all of his attacks without any issues, but the problem of course lies with a prolonged fight making things quite impossible for me who relies on a fast dodge and not tanking (don't have stats to tank).
Ah. I used the pillar a bunch when I fought him tried to keep it as much in between me and him as possible and just poked at him. Worked out okay on my first try solo no less although I'm also not doing hard mode like you.
Killed him before water becomes an issue?
I must have because I don't know what you're talking about there.