I also have this problem.
I am running the Plex add-on on coreelec 19.5 matrix rc1.
I also tried the version from this repository
Same problem.
I then tried both versions of the 19 add-on from the first post in this thread
Same problem.
So it seems this button only works in kodi 18.
A good workaround would be the “up next” add-on but unfortunately that also doesn’t work within the Plex add-on.
Someone suggested having something similar built in and the dev @panni seemed to like the idea.
Hopefully someone will figure something out.
I’m a willing tester if needed.
