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A quick search gives a regular pattern in the form of [trisyllabic laxing](https://en.wikipedia.org/wiki/Trisyllabic_laxing)

> Trisyllabic laxing, or trisyllabic shortening, is any of three processes in English in which tense vowels (long vowels or diphthongs) become lax (short monophthongs) if they are followed by two syllables the first of which is unstressed.

Pronounce -> pronunciation is one of the examples given in the article.