That's a valid concern, I think just like with any other software you need to write tests for the AI model to constantly check if your prompts are working as intended. Basic unit tests would work well in this case.
You are right that fine tuning would probably help to minimize the risks, but it probably never can be zero. New tests will also be needed when customers find new edge cases that break our assumptions.
Testing LLM prompts is a new paradigm that we'll have to learn to deal with.