Expect Joyous Journeys Ahead in Mists of Pandaria Classic
When: 18 November–9 December
The mystical lands of Pandaria are swirling with danger. For our stalwart champions in World of Warcraft Classic progression (currently Mists of Pandaria Classic), take advantage of the Joyous Journey’s experience buff to enhance your odyssey through Azeroth, increasing experience gains by 50% up to level 90!
Create a new character, finish leveling a forgotten one, or invite friends to join you in legendary Pandaria. The Joyous Journeys buff is active by default; however, you can visit any innkeeper in the capital cities and Dalaran to turn it on or off, and it is not available in modern World of Warcraft®, Legion Remix, or World of Warcraft® Classic Era.
Seems like they may have accidentally activated the wrong version, you could report it as bug to have it looked into.