> As a result, the model isn’t trained on understanding the useRalativeImagePath token, and so it outputs something that isn’t a valid token.

That isn't how LLMs generate tokens. Each step outputs a logit for each possible token in the tokenizer (100k in the case of GPT-3.5), then softmaxes the logits to covert them into probabilities, and samples from them depending on temperature to get the token to be used.

It's possible something in the tokenizer BPE merge process breaks due to the rare token, which can be verified offline using tiktoken. But if GPT-4 works, and since GPT-3.5 and GPT-4 use the same tokenizer, then that's likely not the issue.