Collectively spotted this one on Reddit, here (https://www.reddit.com/r/RimWorld/comments/7d16kd/ai_persona_cores_in_a18/).
QuoteFor science, I just used replace all trade ships to call in 26 exotic goods traders and none had one plus called in exotic traders from outlands and came up empty (no mods). This falls in line with my previous experiences: in my last run to go to space I was calling in exotic goods traders to buy gold basically every time it was available and over 237 days of of play I never saw an AI core on an exotic goods trader. Obviously not trying to questioning you, but there really might be some sort of bug with that in A18 or the odds of seeing one are really low.
I think they moved it to the world event. Now you just get the one core. No psychic ships give it either.
Yes, it's intentional. Now that we have a category of unique quest rewards, the AI core has become one of them. It only ever wasn't because we didn't have a better way to gate it than "pay a pile of money". Thanks for reporting.