There is a plexmediaserver-plexpass plugin which might track these plexpass releases more closely.
I am both a free and plexpass user and my experience is that the FreeBSD ports versions tend to be updated at the same, or within a short period of time of each other (the current port maintainer is the same for both the free and pass version of Plex on ports, so they tend to be released at the same time or very closely one following the other). My experience with the pkg version is that it usually lags 2-3 weeks behind a release, but it can vary.
When I run my Plex web interface, it announces a new version 1.15.1.780 is available for download and has been doing this for a week. I went to the web site and discovered that this is because I am using the PlexPass version which makes "Preview" versions available. My apologies.
No worries, we are all here to learn.
Plex does offer the ability to use the Beta update channel on both clients and servers (Settings > General >Server update Channel on the FreeBSD server version accessed through the Windows or local web client). I haven't noticed any settings for a 'Preview' version (maybe on the app.plex or a Mac version?), so I am curious where that setting is. I don't use a beta/preview version on my NAS nor would I recommend it for most users.
I have signed up for beta on my Nvidia Shield client/server....But, both can be quite buggy (which as a person who did QA for years, I'm willing to tolerate and work with, but it should be avoided by anybody unwilling to create bug reports and work with the devs to help understand/test issues).
FYI: I don't hold the bugs against the Plex or other devs. They are creating a significant variety of both client and server versions for various hardware/software/OS's, and that gets complicated. Same goes for ports, pkg, and plugin maintainers as much of this is done by volunteers. And lastly, the FreeNAS devs who work to give us an appliance to bring it all together in a usable way....[steps off soap box]
So, the short answer...
If you always want the latest...use ports to install.
If you're willing to wait a few weeks....use pkgs to install.
If you want the convenience and you're willing to be patient and update every quarter...use the plugin.