nahiri harbinger not as expected

hawkyh1

was playing against tez. used nahiri's 3rd ability
harbinger to reinforce twice my serene steward
to berserker kill baral. with giant spectacle I get
a 14/13 serene steward versus a 10/10 baral.
end of my turn I end up with a 14/3. after
harbinger wears off my serene steward dies. is
this correct? I was expecting steward to survive,
like how arlinn's 1st ability would work?

HH