The OMV Plex Plugin has nothing to do with the Plex Updates itself. Infact the Plugin hasn't been updatet since November last year. (Not negative!)
The Plex Server updates with the files from the Public Repo by Plex Inc. and they don't release Beta Builds to these Repos. So the last stable release is 18.104.22.16823 from 6. December 2017. You can check this yourself by typing this in your shell:
If you want a Beta Build with the Collections then just download the deb file and install it with dpkg -i xxx.deb manually.