Looks like the version of the dictionaries embedded here is 0.7, while there is 0.9 available at http://elspell.math.upatras.gr/files/ooffice/el_GR-0.9.zip
Please update it.
Thanks in advance for your work.
Andras: one for you?
For 5.0: https://gerrit.libreoffice.org/#/c/18754/
Thank you Andras for your very fast response and fix! :-)