Back and PreviousMenu are not the same thing. With the old OSMC remote setup without a custom keymap the home button in that add-on would receive the “previousmenu” action and the return key would send “back” and the Plex add-on has different function for those actions. What you did with your keymap is make both keys send the same action. If Keymap Editor is doing something stupid you will have to manually edit the file.
If you are on the main home screen the back button goes to the top menu and then if you press it once your on the top menu it asks if you want to exit the add-on. If you are several levels down the back button will either go to the previous menu or pop up the top menu with the home icon selected so you can just hit OK to go back to the home screen of the add-on.
I understood your request. This does not exist in Kodi or OSMC.
So am I. I can reproduce it and I know why that is happening.
The fix will come in the normal update channel in the form of an updated custom keymap. You can add the keymap I posted above to fix it for now and it will not cause an issue or have to be removed later when the update comes.
Correct