simulacrums are taking damage and dying when wearing 6 piece masquerade of the burning carnival ptr 22 for a season necro. Is this a know bug?
Thats no bug. Its a feature 
But this time for real. Look at your 2 Piece Bonus and then look at the runes afterwards , and you will be enlightend
It’s a known feature not bug.
Simulacrum’s “Self Sacrifice” rune:
“If you would die with a Simulacrum active, instead the Simulacrum is destroyed and you are fully healed.”
It procs after your cheat death passive (which you should be using) so if that popped and you can’t avoid damage after that / heal up, you know you will lose your sims when you take fatal dmg again - just recast them.
But you shouldn’t really die unless you try way harder content than your gear / paragon / gems allow, or you don’t pay attention to stuff on ground and sit in multiple arcane beams / molten / ice with bad resists etc, or don’t avoid telegraphed attacks that have OHK potential like some charges or wind-up animation of specific enemies / bosses.
Thanks for the comment Anelyn. You are right - i was trying to get ahead before it was time. As I have leveled, their (death and mine) has lessened.
And the death of your enemies has strengthened ^^
Aye I know the feeling is easy to get too caught in the moment and forget the basics, then wonder how did this happen I was getting that a lot last season with my WD because I forgot to keep Soul Harvest stacks up, and boom out of nowhere I was exploding while earlier all incoming damage was manageable