The summons scale their level with the summoner, so a level 18 summoner summons a level 18 summon, for instance.
As summons are leveled, they also gain stats for those levels and that includes initiative and movement speed.
And as you noted, if you summon something that would go above the summoner in initiative (also reflected in the turn queue), it'll only get its first turn on the next actual turn, as the queue can only go from top to bottom (left to right as displayed in-game).
I think this is as intended.
