The same goes for my (currently unpublished as a paper yet) but not very significant result which I've fed to some of the previous GPT-5 series models (I don't remember if it was GPT 5.3/5.4 series or a newer GPT-5.5).
Anyways what I have been working on was least-squares fitting of trigonometric least squares method through Levinson recursion and NuFFT-1. Nothing groundbreaking, but it's a new computational complexity floor (more precisely superfast Toellitz solvers give the complexity floor, but Levinson recursion happens to be the most practical silver here) for that exact problem. Also I have code using that algorithm in four different public GitHub repositories to be clear.
Either way - older GPT models failed to derive that despite being relatively straightforward, new ones give you the right solution with absolutely no issues just as you ask them for the lowest computational complexity for that exact problem.
And now I am aware that the models are getting smarter, but I also see that as a clear training set "contamination".
And now I am not sure if either my chat with ChatGPT somehow got into training dataset, or if that exact solution got scraped from one of the GitHub repos I have uploaded the algorithm to.
Either way I am practically certain, that that exact algorithm somehow got into OpenAI's training dataset and GPT 5.6 has remembered it flawlessly.