Maybe this is a case where more training data isn’t better. There is probably a lot of bad/old C++ out there in addition to new/modern C++, compared to Rust which is relatively all modern.

Yes, I think that's it. There is a lot of horrible C++ code out there, especially on StackOverflow where "this compiled for me" sometimes ends up being the accepted answer. There are also a lot of ways to use C++ poorly/wrong without even knowing it.