I just figured it out.
If you get the Proving Grounds message “While in this area, PVP in enabled. Do you want to accept?” and then click ‘Always’ then the window won’t ever come back, even if you log in and out.
This is what you do to re-enable it:
Go to Settings -> Gameplay
Find ‘Battle Standard Settings’ and set it back to “Always Ask”
And that’s it. Leaving this here in case anyone else has the same question later on. 