I have the same problem. Wife is watching something on our nvidia shield, and it’s not showing up as playing in the dashboard. Network traffic still shows up though. But it doesn’t save progress.
Some times it will show up in the dashboard, a lot of the time it won’t though.
It has only happened on wireless client (nvidia shield) and not on wired (Linux desktop app) for me, both locally.
Server is wired debian box, running plex in linuxserver docker container.
Server version 1.19.1.2645
Nvidia shield and plex app also on the latest software.
Here is the same thing: