Still a bug, but ????
I am using:
GNU Octave, version 4.0.0
Copyright (C) 2015 John W. Eaton and others.
This is free software; see the source code for copying conditions.
There is ABSOLUTELY NO WARRANTY; not even for MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE. For details, type 'warranty'.
Octave was configured for "x86_64-pc-linux-gnu".
OS: ubuntu 16.04 - 64 bit
When I run the code you posted, things that are different from what you have, occur:
(1) The text line only produces centered text if I use
text (2, 0.5, "\lambda is a greek letter", "horizontalalignment", "center")
ie the x location must be within the frame generated by the plot command, even with the "horizontalalignment" modifier.
(2) In that case, the string \lambda appears on the screen as "lambda", however if the double quotes are replaced by single quotes:
text (2, 0.8, '\lambda is a greek letter', "horizontalalignment", "center")
the \lambda produces the greek character. I conclude that in qt the generation of the proper greek character is determined by the use of single quotes rather than double quotes. Notice that in the latex file included in your message there is no text, presumably because of point (1).
The first and last lines of the latex files generated on my system are as follows:
% Title: glps_renderer figure
% Creator: GL2PS 1.3.8, (C) 1999-2012 C. Geuzaine
% For: Octave
% CreationDate: Sun Jul 17 16:47:04 2016