> ‘When two implementations support the same notional feature with slightly differing semantics, should the committee use undefined behavior to resolve the conflict so no users have to change code, or should the committee push for well-defined behavior despite knowing it will break some users?’

That's a false dichotomy. You could also use implementation defined behaviour. Or specify that these n behaviours would be valid.

'Undefined behaviour' is too big of a sledgehammer.