Unfortunately Donegan & Stampe are a phonology, not phonetics, paper and don't reveal where or how they got such data. I also found Parker Jones (2018), a more descriptive phonetics paper, who transcribes all vowels following nasal consonants as nasalized, though he is describing a native speaker who has non-native-speaking parents and acquired the language in immersion schools so this disagreement may be due to a dialectal or diachronic variation. Or have I misinterpreted Donegan & Stampe or Zsiga?
Is it true that (studies have found that) vowels in Hawaiian are never nasalized (in other words, nasal consonants do not cause adjacent vowels to be nasalized)? Are there any other languages where this is the case?
Is it true that oral vowels in French are never nasalized? Is it common for languages with phonemic nasal vowels to never nasalize phonologically oral vowels?