Why might using Kafka introduce eventual consistency bugs that a direct REST call wouldn't have?

Because the producer's job is considered 'done' the moment the event is published, not when every consumer has processed it — there's a window where downstream systems haven't caught up yet. A UI that reads from a downstream service's data right after triggering an event can see stale state until that consumer finishes processing, which needs to be designed around (e.g. optimistic UI updates) rather than assumed away.