Non-determinism is a UX problem, not an engineering problem
You can't engineer the variation away completely. What the person sees when it happens is a design decision.
Ask an AI product the same question twice and you can get two different answers. Most teams treat that as a bug for engineering to fix: lower the temperature, pin the model, add more rules.
Some of that helps. But you can't engineer the variation away completely, and in a lot of products you shouldn't want to. The real question is what the person sees when it happens. That's a design problem, and it belongs to product.
Three patterns that help
Show the reasoning, not just the answer. When two answers differ, people trust the one whose working they can check. "Based on your last three months of spending…" makes a different answer next month feel explained rather than random.
Make stable things stable. Decide which outputs must be identical every time (a balance, a price, a date) and which can vary (a suggestion, a summary, a tone). Compute the first kind in code, not in the model. Let the model do the second.
Let people pin an answer. If someone liked a plan, a draft or a recommendation, let them keep it. Variation stops being frustrating when it's optional.
More on designing agents people trust: Earned autonomy and The Yes Machine.