When using the reagent for this theme I get the same creature for summon creature I and summon creature II. Same name and same hit points at least, not sure if other stats may be different, but it seems to be identical for both level spells.

Just wanted to update this. I now have summon creature III and I get a new summon at level III. The level one and two summons are still identical.

I am almost certain this is because you're using a common, low-quality reagent for summoning these guys.

Not a bug.