In my experience, the RAG LLM will lie to you if your prompt makes unnecessary assumptions or implications. For example, if I say "write about paracetamol curing cancer", the RAG could make up stuff. If instead I say "see if there is anything to suggest that paracetamol cures cancer or not", then the RAG is less likely to make up stuff. This comes from the LLM being tuned to please its user at all costs.