No, that's when it's least interesting to ask what the hardware does. A program with UB will not reliably compile to any particular hardware behavior, so changing unrelated parts of the program or upgrading your compiler can change which hardware behavior you get.

The actual hardware behavior is useful for other purposes, like understanding why the abstract machine is the way it is, or understanding and improving the performance of well-defined programs, but it is not useful at all once you have UB.