how to deal with octave check and tags ?

[Gilles THIBAULT]

I don't know if it will lead to changes in the GDP pitches section, but a 
few things are nebulous for me about pitches.
Here is the problematic code :

%%%%%%%%%%%%%%%%%%%
\version "2.11.34"

frag = \relative c'' {
g'2 b4 d
\tag #'oboe { \octave g''  }
g1
}

{
<<
\new Staff \keepWithTag #'flute \frag
\new Staff \keepWithTag #'oboe \frag
> >
}
%%%%%%%%%%%%%%%%%%%%

I expected a g'''1 for the last note of the flute .
What is my mistake ?

Gilles