Korandje ạ usually derives from *ar or *or. In several inherited Songhay words, however, ạ seems to derive from *a not followed by *r: thus kạṣ-əw "rough" < kas-ow, bạzu "skin bucket, waterbag" < baasu, hạmu "meat" < *hamu, kə̣kkạbu "key" < *karkabu. Yet *a otherwise usually yields a in similar contexts: contrast gani "louse" < *gani, akama "wheat" < *alkama, dzam-a "do it" < *dam-a. It looks as though the vowel in the following syllable is what makes the difference: if it's rounded, you get ạ, otherwise you get a (though one or two exceptions suggest that the story may be more complicated: notably, "difficult" is gab-ə̣w < *gab-ow.) Assuming this rule, *gaadu should regularly have yielded gaaru in mainstream Songhay and gạḍu in Korandje.
What we actually get, however, is gạḍi. Why? Well, Korandje has a rule of final high vowel deletion phrase-internally: if a word ends in i or u, its final vowel will be deleted unless it comes before a pause, ie most of the time. (Basically the opposite of Classical Arabic.) In a number of words, this seems to have led to confusion between original -i, -u, and consonant-final words. For instance, ạṣạnkri "skink" comes from Berber asrmkal, which should regularly have yielded ạṣạmkər; the i is unetymological (Souag 2015). In effect, speakers must have been hypercorrecting final high vowels - a fact which suggests that, if Korandje survives, it may be on its way towards phonologically losing them altogether, much as Classical Arabic did with final short vowels.