2nd time in two days this has happened, someone else mentioned it's happened to them too.

Only main combat slaves have made the transwarp; wild slaves, trade slaves take the whole route galaxy by galaxy.

Not 100% sure how to reproduce this one, it's not 100% guaranteed to happen but it happens fairly often.
First time I noticed was going from peri to HQ after killing a hive, the 2nd time was going from HQ to sol after undocking main CS's and setting the rest to follow.