This isn't an experiment a consumer of the models can actually run. If you have a chance to read the article I linked, it is difficult even for the model maintainers (openai, anthropic, etc.) to look into the model and see what it actually used in it's reasoning process. The models will purposefully hide information about how they reasoned. And they will ignore instructions without telling you.
The problem really isn't that LLM's can't get math/arithmetic right sometimes. They certainly can. The problem is that there's a very high probability that they will get the math wrong. Python or similar tools was the answer to the inconsistency.