I think the issue here is that it's hard work for a native English speaker to keep track of the correctness of every single vowel sound because in English so many are elided or become "uh".
There's a massive variety of vowel sounds in English: Sydney, Irish, Boston, Indian, etcetera.
English speakers can often hear the differences, and many people can produce the different vowels when mimicking the accents (country, city, person, foreigner).