KAFKA-9000, I uncovered a significant flaw in the implementation of foreign key joins. The join only works if supplied with a queriable store. I think this was simply an oversight during implementation and review.
It would be better to fix this now before the release, since the restriction it places on users could represent a significant burden. If they don't otherwise need the store to be queriable, then they shouldn't be forced to allocate enough storage for a full copy of the join result, or add the changelog topic for it either.