Mercer had a great chin when he was younger, although his punch resistance faded as he got much older, as what happens to most guys who fight way too long.
Sanders took absolute bombs off Vitali but was easily dropped by guys like Nate Tubbs. Briggs was the same. Took a beating off Vit, got dropped hard by Wilson.
You're touching on a controversial phenomena known as chime; chin-prime. I've seen Vazquez-Wright. Was Wright pre-chime?
Some guys have great chins from the start, other guys as you said seem to have a chin prime, that sometimes doesn't develop until they get a little bit older, and then they become absolute granite... it's a weird phenomenon.