Unfortunately it's not necessarily specified what counts as "an optimisation". For example, the (DSP-related) compiler I worked on back in the day had an instruction selection pass, and much of the performance of optimised code came from it being a very good instruction selector. "Turning off optimisations" meant not running compiler passes that weren't required in order to generate code, we didn't have a second sub-optimal instruction selector.
And undefined behaviour is still undefined behaviour without all the optimisation passes turned on.
And undefined behaviour is still undefined behaviour without all the optimisation passes turned on.