I'd prefer the phrasing "UB from safe Rust".

The UB is† always in the Unsafe Rust, but that's not necessarily a problem, the problem was that we caused it from our safe Rust and that's definitely not OK.

† Soundness bugs are known to exist in Rust, but for the known ones you have to be really asking for it, so it's not plausible that they'd impact you by accident.