Mismatches between font versions on Google Fonts and on GitHub/NPM packages
Google Fonts still has the old (2.6) version of IBM Plex Serif which lacks Greek support, among other changes, compared to the 3.001 version available on GitHub and via NPM.
Conversely, what Google Fonts offers as IBM Plex Sans (combined variable+static) internally has the 3.201 version for both static and variable files, while the GitHub/NPM releases are 3.005 for the static version (@ibm/plex-sans) and 3.000 for the variable version (@ibm/plex-sans-variable).
Apparently Google Fonts applies their own versioning system to fonts in their library. That is out of our control unfortunately. Similar to how often they choose to update their library with the latest Plex releases.
I recommend to file an issue at Google Fonts if this is a concern to you.