You've got a good close up pic but I wish I could see it even closer...!
From that I can see, there has been some movement/separation in that area, what cause it, can be a number of things, I would start with how dry the wood was when stabilised as it seem to me that, there is some shrinkage around that darker solid/denser spot, and that is normally what the wood do when put through different processes and is not totally dry.
Many times, is the heat exerted when polishing that makes the soft and moist areas of the wood to shrink, only because was stabilized, doesn't mean that the stabilization/juice and cooking went perfect, sometimes we only find out that something wasn't/didn't go right when we are at the final stages of finishing a pen, the polishing/shining of the surface, shows the imperfections a lot better...!
Whatever caused that shrinkage around the dark wood dot, can be fixed and you wouldn't know it was ever there. Would I repair it straight-away...??? no I wouldn't, I would put that barrel a side in a well ventilated place with some mild heat around it, let it stabilise for a bit and then tackle the repair.
I would use the 5 cps CA glue to soak those gaps as the barrel is (CA coating on), do it from each end, you don't want to create air bubbles inside the gaps. When no longer soaks, put it a side overnight, in a warm place.
I would try to remove all the CA over the wood very gently, maybe use 120 sand paper to remove that list bit, re-sand to your normal grits and use the same 5cps CA to coat the blank all around, with the lathe OFF, rubb it as smooth as you can, use a little accelerator and with the Lathe OFF still, sand the barrel with the grain, until any ridges are are gone, do not go all the way to the wood, that will be you "sealer" coat.
Use 240 and 400 grits to get it ready for the extra CA costs, proceed with your normal CA finish processes and the wood should be flawless as the area in question has been "welded" back to the dark spot...!
This is what I would do, off-course...!
Good luck...!
Cheers
George