The Un’Goro version is a 2/1, but the Core version is a 3/1!
That is not a bug. Some core cards have balance changes applied to them when they go into Standard.
https://bnetcmsus-a.akamaihd.net/cms/content_folder_media/W05YUCW6S4K81707418379353.png
It is a bug that the versions are different. When cards entering Core receive buffs, they are meant to apply to all versions of the card.
Can you give an example of another card getting a buff while entering Core whose version in its non-Core set is different?