A little bit in t a book to set up a future series is fine. I have been reading the expanse and they (the writers is really two people) puts an occasional side story that set up shorts story the add to the series. The side stories develop existing characters and have value on their own, not to mention they are relatively short.
Jordan on the other hand went on thousand page rants with side stories that did virtually nothing to develop the existing characters or advance the main story line. That is why I was tired of the series and only finished because I had invested to much time not to finish. Still not sure that was the best choice.