To confirm after additional testing with SimpleX and Plex.
The (GO) button for pause and play works (mine is an old Beo4 ie without any joystick) on Plex.
The (GO) button does not work for pause and (restart) play on SimpleX; however it does work for selection of menus.
I did have to force a lost connection between SimpleX on the ATV4 and the MacMini running the Plex host/server.
How I did this for sure I do not know, but whenever Apps on the ATV4 start misbehaving I double click the home button on the Siri remote (seemingly no Beo4 equivalent for that !) and the flick off (ie cause the Apps to terminate being suspended in the background and therefore release memory). This seemed to have contributed to SimpleX doing a proper reconnection.
BTW and FYI I was happily switching between Plex pauses and SimpleX restarts (and vice versa) of the same movie with barely a frame drop -- which should be expected seeing as it is the MacMini Plex host that is storing the bookmark automatically.