See https://en.wikipedia.org/wiki/Navier–Stokes_existence_and_sm...
“The first statement (A), which is known as the "smoothness" problem, states that there should always exist smooth and globally defined solutions to the Navier–Stokes equations in three-dimensional space. The second statement (C), known as the "breakdown" problem, states that there should be at least one set of initial conditions and external forces for which there are no smooth solutions to the Navier–Stokes equations.“
Edit: Though I notice now the OpenAI blogpost does claim to solve the “smoothness and existence problem”, so maybe there’s simply terminology differences here. I’ll note many mathematicians such as Dennis Sullivan suggest part (A) is the more important part.