Although signed integral type representation is unambiguously defined in terms of unsigned representation, unsigned integral type representation is never specified any further than the bit-width and padding bits being implementation-defined. It is not clear that the ambiguity in the representation is intended to be unspecified. Notably, all other fundamental types are either defined to have the same representation as one of the integral types, or to have implementation-defined representation.

I believe the intent is that all unspecified behavior be explicitly stated to be unspecified, so this amounts to under-specification.