How this works is the LLM predicts the probability of the next token and then an arithmetic coder turns that probability distribution into bits. So it will never hallucinate. In the worst case, when the LLM makes an outrageous prediction, you just use more bits, but it doesn't affect correctness.
Presuming the software is implemented correctly, that can't happen (per the definition of "lossless"). I can imagine this happening with a careless implementation, e.g. if circumstances conspire to allow a slightly different version or configuration of the LLM to be used across compression and decompression.