Is brand drift the same as a model hallucination?

Not exactly. A hallucination invents a fact. Drift can be factually fine and still sound like a different company.

Does a style guide PDF prevent it?

Only if someone pastes the right parts into every chat. A profile the tool reads is harder to skip.

What is AI brand drift?

AI brand drift is output that no longer matches the business. The voice changes, the colors change, or the offer changes because each person pasted a different brief into the model. One saved identity is what stops the next chat from starting over.