I am running into this exact same issue… Took forever to track down. I have an AI with 4 Stimuli, and under different conditions, Hearing will be in different spots in the array. It is authored at Array Position 1, but can show up in 2 under certain circumstances.
A quick fix that worked for me ( but have zero idea why…), was to place one of the AI in the level manually, rather than rely on them all being spawned in dynamically. Funnily enough, just spawning 1 in manually fixes all the dynamically spawned ones.
I hate using things that rely on something being in a particularly numbered array entry. This should 100% be a map, or at least the FAIStimulus Struct should expose the Type member to blueprint so we can just iterate through them all and see which one is which.