No, generally speaking OpenAI doesn't re-use training data between customers. It's worth it to them anyway because they learn what does/doesn't work on different tasks
Of course, it isn't your IP free and clear either, because the base model isn't open so your fine-tuned model will always live inside OpenAI's walled garden.
If you're interested in reinforcement learning on top of truly open models where you own the end product, we're putting a lot of thought into that and are also looking for design partners! Feel free to email me at kyle@openpipe.ai.