In my well-informed, expert opinion backed by decades of experience, it would have been best to add this wording:

"When size is zero, the realloc function shall free the original object, regardless of whether allocating the new object is successful, and thus regardless of the value returned."

With a footnote explaining the ambiguity that exists otherwise, and that existed historically.

A small change in some implementations here would be better than taking a wrecking ball to defined behavior.