But your prompts are not the only inputs. Among other things, there is a random seed injected by the vendor.

That is a primary source of non-determinism.

Then, of course, is the fact that you don't personally have an old copy of the model, and the vendor isn't going to keep the model forever, and there are no unit tests to make sure that, faced with prompts like you gave it before, the newer models won't suffer major regressions in the functionality you were using.

And even if there were no non-determinism, the models suffer greatly (much more so than traditional compilers) from the butterfly effect.

It is literally impossible to pin down part of your prompt in such a way that it always will contribute to good outcomes, and such that you can simply vary a tiny bit of the prompt to logically correlate with tiny variations in the output.