I was thinking the same, and I'm going to go with yes. I had Heaven's Eye materia in my list the last 2 days on Kobolds, but none today, seems like it's a random sample of any 2ndary stat materia.
It doesn't seem to be totally random. I have started to notice there is specific sets of five you get to choose from. I'm not sure if the particular set is tied to particular quest, but all of these predefined sets seem to include somewhat rounded up selection for all sorts of roles. That is, no set is purely caster centric, or purely melee centric, even if not each set includes the desirable one you'd want.
It's probably based on which quests you get. The 3 quests you get are "randomly" chosen out of a pool of like 10 and the materia spread seems to be the same for each tribe.