Let me approach this another way. The idea of a maturation plot occurs in more than one of the various schemas for classifying plots by type. Those schemas divide plots into multiple types, but there are different numbers of types in each.
Heros's journey, on the other hand is not part of a classification schema for plots, it is proposed as the archetype of all plots. If we accept this as true, then all of the plot types in the various plot classification schemas, including the maturation plot, are all hero's journey.
So, if someone proposes that all hero's journey plots are maturation plots, they are saying one of two things:
All plots are maturation plots, since the hero's journey is a maturation plot and is the archetype of all plots. Therefore there are no other types of plot other than maturation, and therefore the answer to your question is no.
There are various types of plots, maturation is one of them, and the hero's journey is a maturation plot, therefore the hero's journey is not the archtype of all plots but just another name for the maturation plot. In that case, the answer to your question is to choose one of the plot typing schemas that includes the maturation plot and pick from all the other types it offers.
There is, of course, a third alternative, which is that John Truby is wrong and the hero's journey is the archetype of all plots and the maturation plot is only one type of hero's journey. One test for this would be to decide if the instances of post-hero's-journey plots that you cite actually fit the hero's journey archetype or not.
I am not familiar with all of them, but Iron Man 2 certainly seems to fit the hero's journey, though perhaps with an incomplete arc. It begins with Stark the hero of the community but facing the ultimate exile of death because the suit is poisoning him. Then comes the call to adventure when he is attacked by Vanco. The wise man appears in the form of Fury. Stark receives a gift that makes his quest possible--his father's reactor plans. He undergoes an ordeal and saves the community. But the arc in incomplete in the sense that he is left exiled from Shield. This is to set up the following films, of course. But it is absolutely classic hero's journey stuff. So, not post hero's journey at all.
Part of the issue here may also be what we mean by maturation. In a classical tribal society, maturity had a clear definition. You were mature when you were ready to take on adult responsibilities within the community. But in a society so individualistic that few people regard themselves as having any responsibility at all to the community, the definition of mature is hard to define, and we do see many adults living what are essentially extended childhoods. In such a society, it would be easy enough to define all personal development as maturation.
And if you do that, then all plots do indeed become maturation plots (unless you ascribe the McKee's assertion that people don't change and that a story reveals character rather then changing it). But if that is the case, you are not redefining the hero's journey or any of the common plot classification schemas, you are simply redefining maturation. And if that is what you are doing, then the answer to your question is again no, since there are no plots that are not maturation plots. (This is clearly not what you are saying, but it is part of trying to account for the view that you ascribe to John Truby.)