Neither .txt not .odt or .doc files are sorted by word count when using this option. The file sorting order does not change. Dolphin 17.12.3 on Kubuntu 18.04 LTS LibreOffice 6.0.7.3 Kate 17.12.3
Yes, this seems broken. I think that .txt files do not support the word count properties, though.
Can confirm this is still an issue on KDE Neon Unstable.