Run with

  /usr/bin/emacs -Q -l init.el bug.org

Then execute the code blocks by using C-c C-c in code block.

Unexpected output (as it includes a blank line ": " and "octave >")
#+begin_src octave :session oct1 :results output
  x = 1;
  x = 1;
  x = 1
#+end_src

#+RESULTS:
: 
: octave> x = 1

Unexpected output (as it does not include ": x = 1")
#+begin_src octave :session oct2 :results output
  x = 1;
  x = 1
  x = 1;
#+end_src

#+RESULTS:

By comparison, these code block give expected results
#+begin_src octave :session oct3 :results output
  x = 1
  x = 1
  x = 1
#+end_src

#+RESULTS:
: x = 1
: x = 1
: x = 1

#+begin_src octave :session oct4 :results output
  x = 1;
  x = 1
  x = 1
#+end_src

#+RESULTS:
: 
: x = 1
: x = 1

Also, if the ":session" headers are removed the all the code blocks give the expected results, for example,
#+begin_src octave :results output
  x = 1;
  x = 1;
  x = 1
#+end_src

#+RESULTS:
: x = 1