When playing halo wars 2 I liked to select my team and enemies to be on a random leader. However recently when I choose random leader all the AI and myself are never chosen as my bought DLC leaders. I have done around 10 matches trying to get a DLC character to appear but it seems to only be the non 6 main leaders.
I too have had a similar experience where specific heroes aren’t included, except it pertains to unlocking Blitz cards. This happened after re-downloading the Hero DLCs to fix an alternative bug.
I encourage trying to re-download your Hero DLCs to see if the issue fixes itself that way. Be warned however, other things might break. I’m still figuring things out.