Once you add the suffix, the stress shifts to the one syllable before the suffix.

一旦你加了后缀，重音就转移到后缀前的一个音节上。

So when it's used as an adjective, the syllable stress shifts to the second syllable and the first syllable reduces right down so the vowel sound becomes a schwa.

所以当它用作形容词时，音节重读移到第二个音节上，第一个音节没有重音了，元音就变成了中元音。