Implementations can define behavior for undefined behavior. The difference to implementation-defined behavior is that for the latter implementations MUST define some behavior (from the set of options specified by the standard), whereas for undefined behavior they don’t need to.
If an implementation has defined some behavior for sscanf undefined behavior, and then the standard defines a different behavior, then the existing implementation would become nonconforming, and an updated version of the implementation would be not backwards compatible with the existing one. That’s why such changes to the standard can be problematic.
If an implementation has defined some behavior for sscanf undefined behavior, and then the standard defines a different behavior, then the existing implementation would become nonconforming, and an updated version of the implementation would be not backwards compatible with the existing one. That’s why such changes to the standard can be problematic.