The stress is on the penult, isn’t it? Here vocalis ante vocalem non corripitur.
It’s like illius, etc.
The only exception I see is Allen & Greenough, 113.c:
https://dcc.dickinson.edu/grammar/latin/1st-and-2nd-declension-adjectives-genitive-īus-dative-ī
The i in the genitive singl. is often short in poetry, so in that case, the accent would fall on the antepenult in alterius.
It is short also in Ecclesiastical Latin, e.g. Gospel of the Mass of the Lord’s Supper the Missale Romanum