The answer is the same as with the previous questions. Microsoft Bing runs on GTP-4 (Prometheus) AI model which is the current peak of OpenAI’s technology.
However, no matter how much better the tech gets, there’s still the underlying problem of training data.
Again, all GPT models are trained on publicly available data which makes them unsuitable to act as reliable grammar checkers.
These LLM-powered chatbots  can find and remove most mistakes but almost alway the most insidious ones remain ready to embarrass you.
There’s a good reason why Grammarly and other paid grammar checkers are still in business despite Bing Chat and ChatGPT being free tools and available worldwide.