question about ottava spanner

[Jinsong Zhao]

Hi there,

In the following MWE, you may notice that the ottava spanner have different 
behavior. It may be my fault. However, I don’t know how to correct it.

The first question is why the first ottava spanner extent to the second note? 
The second question is how to prevent the ottava spanner merging, shown as the 
last two notes.

Any suggestion will be really appreciated.

Best,
Jinsong

\version "2.19.83"

\relative{
  \clef bass
  \key g \major
  \time 3/4
  \override Staff.OttavaBracket.direction = #DOWN
  << 
    { s4 <b e g b>2 }
    \\
    {
      \ottava #1
      \set Staff.ottavation = #"8"
      <b, b'>2
      \ottava #0
      \ottava #1
      \set Staff.ottavation = #"8"
      <b b'>4
      \ottava #0
    }
  >>
  \ottava #1
  <b b'>2
  \ottava #0
  \ottava #1
  <b b'>4
  \ottava #0
}