[jaggederest]

I believe in this case spelling is mimicking pronunciation.

In my accent, at least, I pronounce the phrase 'extr_vert-introvert' with an open A, contrasting to the pursed O of introvert.

But if I say the word 'extr_verts', by itself, I pronounce it with an O, identical to 'introvert'.

It's difficult because for me, at least, in my accent, both are degrading to an unstressed vowel anyway, so it's a fine gradation.