This bug seems to have been introduced in a recent patch. Previously, you could have more than one summon, and their portraits would line up next to the summoner's, as expected.
Now, it seems this is no longer the case.
Incarnate summoned after a follower is attached to the player:
The incarnate works in battle, despite the portrait missing:
Dismissing the follower makes the portrait show up:Video Example
Conversely, summoning the incarnate, and then adding the follower makes the follower's portrait hidden, so it seems to respect whichever character was attached first.