How did “defect” and “defect” come to have different pronunciations?

(This is an expanded version of my earlier comment on the question.)

This is not the direct answer to your question, but anyway it seems pretty relevant.

In English, there are many two-syllable words which have an accent on the first syllable when used as a noun and on the second syllable when used as a verb, and “defect” is one of them. According to web search, such words seem to be known as “initial-stress-derived nouns.”

Wikipedia gives some explanation for this phenomenon:

In English, since the early modern period,^[1] polysyllabic nouns tend to have an unstressed final syllable, while verbs do not. Thus, the stress difference between nouns and verbs applies generally in English, not just to otherwise-identical noun-verb pairs. ^[2] […] When the stress is moved, the pronunciation, especially of vowels, often changes in other ways as well. Most common is the reduction of a vowel sound to a schwa when it becomes unstressed.