The original title guidelines states that TMDb should use the original on-screen title. That means the titles in a series don't have to be consistent. That being said, both titles are in use, and the moderator never explain the change to The Land Before Time: The Great Valley Adventure. If you have evidence that it was released as "The Land Before Time II: The Great Valley Adventure" theater, please share it.

can't we just change it then? Most sites list it as The Land Before Time II: The Great Valley Adventure. Couldn't find a proper archive entry but since all other movies (not just here) have its title written with the number...

Well, I found a vhs commercial where its advertised with the "II" on the box: https://www.youtube.com/watch?v=IKWIWG0f2qk

@banana_girl said:

If you have evidence that it was released as "The Land Before Time II: The Great Valley Adventure" theater, please share it.

I posted evidence it got released as "The Land Before Time II: The Great Valley Adventure" on VHS (see 0:27 in the video above). There is no older footage. There's no proof for the original title to be "The Land Before Time: The Great Valley Adventure" either and the mod who changed it didn't answer to any of the threads.. I think it would be the right call to change it back.